AudioService: handle errors when reconnecting mixes after server crash

  Log errors when reconnecting dynamic policy mixes after an audio
server crash.
  Add IntDef for AudioSystem errors, and conversion to String.
  Add support for notifying an AudioPolicy user that it was
unregistered.

Bug: 133279309
Test: connect DAP, kill audio server, verify no errors reported
Change-Id: I1dbfdac1873ed9c44c20ca9d25bbe01ce9904ee5
Merged-In: I1dbfdac1873ed9c44c20ca9d25bbe01ce9904ee5