* [android] fix map snapshotter marker example
The reference to the MapSnapshotter needs to be held for the duration of the snapshot, otherwise it might be GC'd
* [android] MapRendererRunnable - avoid weak reference table overflow
Apparently on some devices the weak reference table is limited (numbers around 52000). Even though we don't use that many weak references, when GC is not called for a while they can stack up and a crash will occur before the GC has had the time to clear the references. The C++ peer now holds on to a global ref (strong) which can be obtained to queue the java peer and then release automatically so that the GC can take over after the runnable has been executed.
