audiooutput: Serialize access to AudioOutput

AudioOutput can be accessed by NuPlayer and MediaPlayer::Client.
This can cause race conditions if say one of them deletes the AudioOutput
(e.g. on a offload tear down) while other accesses it. Add synchronization
for safe access.

CRs-Fixed: 837500
Change-Id: Ic09f706a3c3f73a93d4dc8477bd4caee208c292d