[android] Add jni binding for styleable snapshotter

[Chaoba]

This pr add jni binding for styleable napshotter.
The related android pr: mapbox/mapbox-gl-native-android#268

[alexshalamov]

I think we need to expose MapSnapshotterObserver and MapSnapshotter::style() to the Java side, so that the client can add / remove layers and sources, as well as subscribe to MapSnapshotterObserver events.

[Chaoba]

@alexshalamov This is the first idea come into my mind, but Style instance is created with NativeMapView in android sdk. @tobrun do you think it is possible to create Style from MapSnapshotter?

[tobrun]

@Chaoba current Style.java is tied-to and optimized-for the map implementation and will not match the requirements for this feature. It's not a peer object that has 1-on-1 relation with the c++ equivalent.

For this iteration of the SDK, I feel we should support the full style API as addLayer(Layer layer)/removeSource()/etc. on the java side that will eventually call into snapshotter->getStyle() in jni code. This way we don't need to marshall the style object.

[alexshalamov]

@pozdnyakov I could not figure out why scheduled lambda goes out of scope in caller thread before it is pushed to the target scheduler queue. Both 1 and 2 were not reliable. 3rd approach doesn't look nice, but works.

1. First approach eeea3af
2. Custom deleter cd91d5e
3. Deleting lambda d0d560e

[pozdnyakov]

@alexshalamov here I guess it shall be weakScheduler->schedule([ptr = std::shared_ptr<mbgl::MapSnapshotter>(p)] {});

[pozdnyakov]

few nits