Use PipMediaController in .tv.PipController

Make .tv.PipController rely on PipMediaController for media session
monitoring and managering.
Refactor .tv.PipControlsViewController to use PipMediaController to
retrieve the media buttons.
Make .tv.PipNotification subscribe to PipMediaController to receive
callbacks about media metadata updates.

Bug: 169575409
Bug: 165795012
Test: make WindowManager-Shell; make SystemUI
Change-Id: I397f93eb4433116ba4f476bda7374b07d97e082d