Disable output switcher based on available routing

The MediaRouter2Manager will tell us if there is available route
information. Disable the output switcher when
MediaRouter2Manager#getRoutingSessionForMediaController returns null.

Fixes: 155266917
Test: atest src/com/android/systemui/media/MediaDeviceManager.kt
Test: manual - Player for YouTube cast session still has disabled output
switcher because route information isn't available.

Change-Id: I95af24cf466a83c158350e9b9702d12d5db4861c