base

Fix leaks due to GC circular references.

The DisplayEventReceiver and SensorManager event queue both get
leaked when the Looper thread they are attached to dies because
the Java object holds a strong reference to its native peer and
meanwhile the native peer holds a strong reference to the Java
object through JNI.

Fixed the issue by indirecting through a weak reference as was
done for InputEventReceiver some time ago.

Bug: 12455729
Change-Id: I3d80a2a190192d1a2981bf5ae0cad30f0f7688a5