Unwrap Context in order to retrieve Activity subclass (#59)

This fixes crash on Expo client which is wrapping Activity prior to passing it as a context to the root view.

After my recent change in the logic on how we access main activity we know extract the reference to it using `getContext` from the root view. Previously we were using `getTopLevelActivity` which wasn't working well in the cases where other non-react-native activities were transitioning in or out. The new approach however turned out not to be the best as for example expo client does not pass activity instance as a context directly to the root view. Instead the activity class is wrapped in ContextThemeWrapper ([see it here](https://github.com/expo/expo/blame/41458d1de91544c41097818db21e44e9aa84688c/android/expoview/src/main/java/versioned/host/exp/exponent/ReactUnthemedRootView.java#L13)).

We now try to unwrap the context if it is not a fragment activity using `getBaseContext`

This fixes expo/expo#3191

android/src/main/java/com/swmansion/rnscreens/ScreenContainer.java

@@ -2,6 +2,7 @@
 
 import android.app.Activity;
 import android.content.Context;
+import android.content.ContextWrapper;
 import android.support.annotation.Nullable;
 import android.support.v4.app.Fragment;
 import android.support.v4.app.FragmentActivity;
@@ -93,8 +94,12 @@ private FragmentActivity findRootFragmentActivity() {
     if (!(parent instanceof ReactRootView)) {
       throw new IllegalStateException("ScreenContainer is not attached under ReactRootView");
     }
-    // ReactRootView is expected to be initialized with the main React Activity as a context
+    // ReactRootView is expected to be initialized with the main React Activity as a context but
+    // in case of Expo the activity is wrapped in ContextWrapper and we need to unwrap it
     Context context = ((ReactRootView) parent).getContext();
+    while (!(context instanceof FragmentActivity) && context instanceof ContextWrapper) {
+      context = ((ContextWrapper) context).getBaseContext();
+    }
     if (!(context instanceof FragmentActivity)) {
       throw new IllegalStateException(
               "In order to use RNScreens components your app's activity need to extend ReactFragmentActivity or ReactCompatActivity");