Add new api for output switcher

This CL include following change:

- Add addDeviceToPlayMedia() to let MediaDevice can play current media
  together.
- Add removeDeviceFromMedia() to let MediaDevice stop play media.
- Add getSelectableMediaDevice() to get the MediaDevice list that can be
  added to current media.
- Add adjustSessionVolume() to let output switcher to change the volume
  of RoutingSession.
- Add getSessionVolumeMax() and getSessionVolume() to get max and
  current volume of session.
- Add ShadowRouter2Manager.java to implement test case.
- Add test case.

Bug: 142912244
Test: make -j42 RunSettingsLibRoboTests
Change-Id: I1bda5a1c3a788778c384adce851fc2ffa8c1aa73