audioflniger: fix leaked audiosession references

acquireAudioSessionId() should not create session references
if the caller is not listed in the known notification clients.
It happens when the MediaPlayer or AudioTrack is created by the
mediaserver on behalf of a client (e.g CameraService). In this case
releaseAudioSessionId() can be called from a different pid and
the session reference will not be removed.
Also we will not be able to remove the reference if the client
process dies.

Bug: 10606426.
Change-Id: Ibb5e27518daeb484a2e66302e4c6221cbaca5b70