[android] - throw exception when another style is loading, correct RuntimeStyleTest when removing layer at, add javadoc to isFullyLoaded

platform/android/MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/style/RuntimeStyleTests.java

@@ -133,7 +133,7 @@ public class RuntimeStyleTests extends BaseActivityTest {
 
         // Test remove by index bounds checks
         Timber.i("Remove layer at index > size");
-        assertNull(mapboxMap.getStyle().removeLayerAt(Integer.MAX_VALUE));
+        assertFalse(mapboxMap.getStyle().removeLayerAt(Integer.MAX_VALUE));
       }
     });
   }