This allows us to shut down the IAudioSessionManager after Cubeb has stopped,
so there shouldn't be STA apartment deadlock issues with it.
IAudioSessionManager's shutdown always requires the STA. This patch eliminates
MTA threads involvement at shutdown, which was previously needed for thread
safety. We keep other operations on MTA (background) threads, for performance.
This also removes a lot of unused functionality. AudioSession hasn't been
needed outside of the parent process since audio remoting landed.
Differential Revision: https://phabricator.services.mozilla.com/D251773
18de707b8e