android: Send network change events from a separate thread via JNI

android: Send network change events from a separate thread via JNI

Doing this from the main UI thread (which delivers the broadcast) might
cause an ANR if there is a delay (e.g. while acquiring a mutex in the
native parts). There might also have been a race condition during
termination previously because Unregister() was not synchronized so there
might have been dangling events that got delivered while or after the mutex
in the native parts was destroyed.