[ios] Refactor MGLMapView's location services

- Pause location services when the app goes to sleep; address iOS 8 blue bar issue.
- Only ask for location permissions if not already granted/denied.
- Cleanup usage of `.isDormant` → `.dormant`

Partially addresses #2945, for which a full and complete fix waits until #4030.

platform/ios/src/MGLMapView.mm

@@ -760,7 +760,7 @@ - (void)setOpaque:(BOOL)opaque
 // This is the delegate of the GLKView object's display call.
 - (void)glkView:(__unused GLKView *)view drawInRect:(__unused CGRect)rect
 {
-    if ( ! self.isDormant)
+    if ( ! self.dormant)
     {
         CGFloat zoomFactor   = _mbglMap->getMaxZoom() - _mbglMap->getMinZoom() + 1;
         CGFloat cpuFactor    = (CGFloat)[[NSProcessInfo processInfo] processorCount];
@@ -906,7 +906,7 @@ - (void)willTerminate
 {
     MGLAssertIsMainThread();
 
-    if ( ! self.isDormant)
+    if ( ! self.dormant)
     {
         [self validateDisplayLink];
         self.dormant = YES;
@@ -957,6 +957,8 @@ - (void)sleepGL:(__unused NSNotification *)notification
     {
         self.dormant = YES;
 
+        [self validateLocationServices];
+
         [MGLMapboxEvents flush];
 
         _displayLink.paused = YES;
@@ -1006,6 +1008,8 @@ - (void)wakeGL:(__unused NSNotification *)notification
         _mbglMap->resume();
 
         _displayLink.paused = NO;
+
+        [self validateLocationServices];
     }
 }
 
@@ -3022,45 +3026,31 @@ - (void)annotationImageNeedsRedisplay:(MGLAnnotationImage *)annotationImage
 
 #pragma mark - User Location -
 
-- (void)setShowsUserLocation:(BOOL)showsUserLocation
+- (void)validateLocationServices
 {
-    if (showsUserLocation == _showsUserLocation || _isTargetingInterfaceBuilder) return;
+    BOOL shouldEnableLocationServices = self.showsUserLocation && !self.dormant;
 
-    _showsUserLocation = showsUserLocation;
-
-    if (showsUserLocation)
+    if (shouldEnableLocationServices && ! self.locationManager)
     {
-        if ([self.delegate respondsToSelector:@selector(mapViewWillStartLocatingUser:)])
-        {
-            [self.delegate mapViewWillStartLocatingUser:self];
-        }
-
-        self.userLocationAnnotationView = [[MGLUserLocationAnnotationView alloc] initInMapView:self];
-        self.userLocationAnnotationView.autoresizingMask = (UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleRightMargin |
-                                                            UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleBottomMargin);
-
-        self.locationManager = [CLLocationManager new];
+        self.locationManager = [[CLLocationManager alloc] init];
 
 #if __IPHONE_OS_VERSION_MAX_ALLOWED >= 80000
-        // enable iOS 8+ location authorization API
-        //
-        if ([CLLocationManager instancesRespondToSelector:@selector(requestWhenInUseAuthorization)])
+        if ([CLLocationManager instancesRespondToSelector:@selector(requestWhenInUseAuthorization)] && [CLLocationManager authorizationStatus] == kCLAuthorizationStatusNotDetermined)
         {
-            BOOL hasLocationDescription = [[NSBundle mainBundle] objectForInfoDictionaryKey:@"NSLocationWhenInUseUsageDescription"] ||
-                [[NSBundle mainBundle] objectForInfoDictionaryKey:@"NSLocationAlwaysUsageDescription"];
+            BOOL hasLocationDescription = [[NSBundle mainBundle] objectForInfoDictionaryKey:@"NSLocationAlwaysUsageDescription"] || [[NSBundle mainBundle] objectForInfoDictionaryKey:@"NSLocationWhenInUseUsageDescription"];
             if (!hasLocationDescription)
             {
                 [NSException raise:@"Missing Location Services usage description" format:
-                 @"In iOS 8 and above, this app must have a value for NSLocationWhenInUseUsageDescription or NSLocationAlwaysUsageDescription in its Info.plist."];
+                 @"In iOS 8 and above, this app must have a value for NSLocationAlwaysUsageDescription or NSLocationWhenInUseUsageDescription in its Info.plist."];
             }
-            // request location permissions, if both keys exist ask for less permissive
-            if ([[NSBundle mainBundle] objectForInfoDictionaryKey:@"NSLocationWhenInUseUsageDescription"])
+
+            if ([[NSBundle mainBundle] objectForInfoDictionaryKey:@"NSLocationAlwaysUsageDescription"])
             {
-                [self.locationManager requestWhenInUseAuthorization];
+                [self.locationManager requestAlwaysAuthorization];
             }
-            else if ([[NSBundle mainBundle] objectForInfoDictionaryKey:@"NSLocationAlwaysUsageDescription"])
+            else if ([[NSBundle mainBundle] objectForInfoDictionaryKey:@"NSLocationWhenInUseUsageDescription"])
             {
-                [self.locationManager requestAlwaysAuthorization];
+                [self.locationManager requestWhenInUseAuthorization];
             }
         }
 #endif
@@ -3069,12 +3059,37 @@ - (void)setShowsUserLocation:(BOOL)showsUserLocation
         self.locationManager.delegate = self;
         [self.locationManager startUpdatingLocation];
     }
-    else
+    else if ( ! shouldEnableLocationServices && self.locationManager)
     {
         [self.locationManager stopUpdatingLocation];
         [self.locationManager stopUpdatingHeading];
         self.locationManager.delegate = nil;
         self.locationManager = nil;
+    }
+}
+
+- (void)setShowsUserLocation:(BOOL)showsUserLocation
+{
+    if (showsUserLocation == _showsUserLocation || _isTargetingInterfaceBuilder) return;
+
+    _showsUserLocation = showsUserLocation;
+
+    if (showsUserLocation)
+    {
+        if ([self.delegate respondsToSelector:@selector(mapViewWillStartLocatingUser:)])
+        {
+            [self.delegate mapViewWillStartLocatingUser:self];
+        }
+
+        self.userLocationAnnotationView = [[MGLUserLocationAnnotationView alloc] initInMapView:self];
+        self.userLocationAnnotationView.autoresizingMask = (UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleRightMargin |
+                                                            UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleBottomMargin);
+
+        [self validateLocationServices];
+    }
+    else
+    {
+        [self validateLocationServices];
 
         if ([self.delegate respondsToSelector:@selector(mapViewDidStopLocatingUser:)])
         {