MergeOfflineRegionsCallbacks are never fired

[Guardiola31337]

Integrating offline maps downstream in mapbox-navigation-android 👀 mapbox/mapbox-navigation-android#1895 we've noticed that when trying to mergeOfflineRegions the OfflineManager.MergeOfflineRegionsCallback listeners are never fired https://github.com/mapbox/mapbox-navigation-android/blob/1ced0224aab1484024af079745498dd401d0882f/libandroid-navigation-ui/src/main/java/com/mapbox/services/android/navigation/ui/v5/MapOfflineManager.java#L58-L68

We were able to reproduce in mapbox-gl-native https://github.com/mapbox/mapbox-gl-native/tree/pg-downstream-module and after debugging it seems that the WeakReferences callbacks implemented in FileUtils

mapbox-gl-native/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/utils/FileUtils.java

Line 20 in d25ae1c

private final WeakReference<OnCheckFileReadPermissionListener> listenerWeakReference;

mapbox-gl-native/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/utils/FileUtils.java

Line 77 in d25ae1c

private final WeakReference<OnCheckFileWritePermissionListener> listenerWeakReference;

are garbage collected somehow and always returning null

mapbox-gl-native/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/utils/FileUtils.java

Line 45 in d25ae1c

OnCheckFileReadPermissionListener listener = listenerWeakReference.get();

mapbox-gl-native/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/utils/FileUtils.java

Line 102 in d25ae1c

OnCheckFileWritePermissionListener listener = listenerWeakReference.get();

which means that the listeners are never fired because when null, nothing is done.

For debugging purposes we've removed the WeakReferences and the callbacks were 👌 Obviously, in order to keep avoiding potential memory leaks, this is not enough, but maybe we could add a setter in the tasks, that could be set to null accordingly, so it's garbage collected without needing WeakReference. Thoughts?

cc @zugaldia