Make RemoteController weakly referenced by IRemoteControlDisplay

In the RemoteController implementation, the inner class that
 implements the IRemoteControlDisplay interface must have a weak
 link to the RemoteController instance with which it is associated,
 as MediaFocusControl and RemoteControlClient hold a strong
 reference to this binder object in a different remote process.
 Without a weak reference, any object referencing RemoteController
 couldn't be independently garbage collected without a garbage
 collection in the remote process.

Bug 8209392

Change-Id: I29e4274c45249b3cb0d3d89417c69e8fe8f62fc4