[android] - add instrumentation tests for FileSource activation/deactivation

platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/storage/FileSource.java

@@ -6,6 +6,8 @@ import android.content.pm.PackageManager;
 import android.content.res.AssetManager;
 import android.os.Environment;
 import android.support.annotation.NonNull;
+import android.support.annotation.UiThread;
+
 import com.mapbox.mapboxsdk.Mapbox;
 import com.mapbox.mapboxsdk.constants.MapboxConstants;
 import timber.log.Timber;
@@ -43,6 +45,7 @@ public class FileSource {
    * @param context the context to derive the cache path from
    * @return the single instance of FileSource
    */
+  @UiThread
   public static synchronized FileSource getInstance(Context context) {
     if (INSTANCE == null) {
       String cachePath = getCachePath(context);
@@ -122,6 +125,8 @@ public class FileSource {
     initialize(Mapbox.getAccessToken(), cachePath, assetManager);
   }
 
+  public native boolean isActivated();
+
   public native void activate();
 
   public native void deactivate();