Add flag for singleton AudioManagerRouteController

To fix the audio routing HSUM related issue.
The flag is under the media_better_together namespace.

Change-Id: I9df888074417003b85a73fdff361dfa94d89c403
Bug: b/372868909
Test: presubmit
Flag: com.android.media.flags.enable_use_of_singleton_audio_manager_route_controller