MediaRouter2: Add getDefaultController()

The controller which can control the system routes (phone speaker,
bluetooth, etc) is returned by this new method.

This CL makes the default controller can know the current system route,
and make SessionCallback#onSessionInfoChanged() be called when
the current system route is changed.

Note that selecting route (BT / speaker) via this controller is not
yet supported by this CL.

Bug: 147937954
Test: atest mediaroutertest
      Also, manually checked logs by connecting bluetooth device.
Change-Id: I8c6254855f8121d1d2331959224fe7237affa1e2