towards MR2: update to new backend

Splitting MediaDeviceManager out of MediaControlPanel. This will make it
easier to integrate
MediaRouter2Manager#getRoutingSessionForMediaController.

Bug: 155266917
Test: atest tests/src/com/android/systemui/media/MediaDeviceManagerTest.kt
Test: atest tests/src/com/android/systemui/media/MediaDataCombineLatestTest.java
Test: manual - switch route between phone, headphones and cast and check
output switcher chip in the player.

Change-Id: Ibe6a7e41bd8fb0f0976d1c1c7f2f94cfc4fc6f5e