[Xamarin.Android.Build.Tasks] use designtimebuild.props

Context: #1933
Context: #1943

One of the issues I noticed while debugging the issue with #1933 is
that `$(IntermediateOutputPath)build.props` gets invalidated if
`$(DesignTimeBuild)` changes. `build.props` triggers alot of targets
to build completely again, so this is pretty bad for our build times
in an IDE...

Design-Time Builds can run quite frequently in VS Windows, and we
don't want to rebuild a bunch of things unnecessarily when the user
switches back to a regular build.

So a solution, is to use a `designtimebuild.props` that works
independantly of `build.props`. This prevents some slower targets from
running when they shouldn't, such as `_UpdateAndroidResgen`.

I added a test to validate these changes, which also verify that
`IncrementalClean` isn't deleting these files.

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs

@@ -120,6 +120,41 @@ public void FooMethod () {
 			}
 		}
 
+		[Test]
+		public void SwitchBetweenDesignTimeBuild ()
+		{
+			var proj = new XamarinAndroidApplicationProject ();
+
+			using (var b = CreateApkBuilder (Path.Combine ("temp", TestName))) {
+				Assert.IsTrue (b.Build (proj), "first *regular* build should have succeeded.");
+				var build_props = b.Output.GetIntermediaryPath ("build.props");
+				var designtimebuild_props = b.Output.GetIntermediaryPath ("designtimebuild.props");
+				FileAssert.Exists (build_props, "build.props should exist after a first `Build`.");
+				FileAssert.DoesNotExist (designtimebuild_props, "designtimebuild.props should *not* exist after a first `Build`.");
+
+				b.Target = "Compile";
+				Assert.IsTrue (b.Build (proj, parameters: new [] { "DesignTimeBuild=True" }), "first design-time build should have succeeded.");
+				FileAssert.Exists (build_props, "build.props should exist after a design-time build.");
+				FileAssert.Exists (designtimebuild_props, "designtimebuild.props should exist after a design-time build.");
+
+				b.Target = "Build";
+				Assert.IsTrue (b.Build (proj), "second *regular* build should have succeeded.");
+				FileAssert.Exists (build_props, "build.props should exist after the second `Build`.");
+				FileAssert.Exists (designtimebuild_props, "designtimebuild.props should exist after the second `Build`.");
+
+				//NOTE: none of these targets should run, since we have not actually changed anything!
+				Assert.IsTrue (b.Output.IsTargetSkipped ("_UpdateAndroidResgen"), "`_UpdateAndroidResgen` should be skipped!");
+				//TODO: I would like to add this assertion, but it requires: https://github.com/xamarin/xamarin-android/pull/1930
+				//Assert.IsTrue (b.Output.IsTargetSkipped ("_LinkAssembliesNoShrink"), "`_LinkAssembliesNoShrink` should be skipped!");
+
+				b.Target = "Clean";
+				Assert.IsTrue (b.Build (proj), "clean should have succeeded.");
+
+				FileAssert.DoesNotExist (build_props, "build.props should *not* exist after `Clean`.");
+				FileAssert.DoesNotExist (designtimebuild_props, "designtimebuild.props should *not* exist after `Clean`.");
+			}
+		}
+
 		[Test]
 		public void BuildPropsBreaksConvertResourcesCases ()
 		{

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -305,6 +305,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 	<_AndroidSequencePointsMode Condition=" '$(_AndroidSequencePointsMode)' == ''">None</_AndroidSequencePointsMode>
 	<_InstantRunEnabled Condition=" '$(_InstantRunEnabled)' == '' ">False</_InstantRunEnabled>
 	<_AndroidBuildPropertiesCache>$(IntermediateOutputPath)build.props</_AndroidBuildPropertiesCache>
+	<_AndroidDesignTimeBuildPropertiesCache>$(IntermediateOutputPath)designtimebuild.props</_AndroidDesignTimeBuildPropertiesCache>
 
 	<AndroidGenerateJniMarshalMethods Condition=" '$(AndroidGenerateJniMarshalMethods)' == '' ">False</AndroidGenerateJniMarshalMethods>
 
@@ -426,6 +427,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 		<_AndroidResourcePathsCache Condition=" '$(DesignTimeBuild)' == 'true' And !Exists ('$(_AndroidResourcePathsCache)') ">$(_AndroidResourcePathsDesignTimeCache)</_AndroidResourcePathsCache>
 		<_AndroidLibraryImportsCache Condition=" '$(DesignTimeBuild)' == 'true' And !Exists ('$(_AndroidLibraryImportsCache)') ">$(_AndroidLibraryImportsDesignTimeCache)</_AndroidLibraryImportsCache>
 		<_AndroidLibraryProjectImportsCache Condition=" '$(DesignTimeBuild)' == 'true' And !Exists ('$(_AndroidLibraryProjectImportsCache)') ">$(_AndroidLibraryProjectImportsDesignTimeCache)</_AndroidLibraryProjectImportsCache>
+		<_AndroidBuildPropertiesCache Condition=" '$(DesignTimeBuild)' == 'true' ">$(_AndroidDesignTimeBuildPropertiesCache)</_AndroidBuildPropertiesCache>
 	</PropertyGroup>
 	<MakeDir Directories="$(_AndroidDesignTimeResDirIntermediate)" Condition=" '$(DesignTimeBuild)' == 'true' " />
 </Target>
@@ -3033,6 +3035,7 @@ because xbuild doesn't support framework reference assemblies.
 	<Delete Files="$(_AndroidLintConfigFile)" />
 	<Delete Files="$(_AndroidResourceDesignerFile)" Condition=" '$(AndroidUseIntermediateDesignerFile)' == 'True' " />
 	<Delete Files="$(_AndroidBuildPropertiesCache)" />
+	<Delete Files="$(_AndroidDesignTimeBuildPropertiesCache)" />
 	<Delete Files="$(_AndroidLibraryImportsCache)" />
 	<Delete Files="$(_AndroidStaticResourcesFlag)" />
 	<Delete Files="$(_AndroidLibraryProjectImportsCache)" />