AudioService: implement de-registration (release) of recorders

Clients must unregister themselves when client-side AudioRecord
goes away. This allows removing the tracking record sooner than
the client app goes away.

"DEATH" event has been renamed to "RELEASE". Client death is handled
the same way as regular release.

Bug: 123312504
Test: start and stop audio recording, check "dumpsys audio"
Change-Id: I6a9578b29ab3c41bac2bf4c823224276efbe1beb