Layoutlib: Properly dispose of bitmap delegate.

This is done by initializing the android.util.Finalizers that's
reponsible for calling out to the native bitmap destructor.
Also implemented the native bitmap destructor

Also fix Bridge by removing obsolete API methods, and removing
some unneeded synchronized blocks now that the whole rendering
(and scene creation) is protected by a synchronized on the bridge
object anyway.

Change-Id: Ie1792da6db354836542dfc11f457fe4a6d78ddfb