SoundPool: Improve single stream SoundPool handling

By design, the StreamManager ramps down volume
on a Stream stop to prevent pops and glitches.
When the SoundPool is configured only with a single stream,
there may be a short period of unavailability of that stream
while stop is called by the worker thread; an immediate
play after a stop may return 0 (failure).

To allow immediate play after stop for a *single* Stream configured
SoundPool, we lock the StreamManager worker thread so that the
stop call is processed and the stream is visible to the client for use.

We prefer not to keep this lock for the multiple Stream case as it
prevents concurrent initiation of sounds from multiple StreamManager
worker threads and such blocking is not appropriate for games.

Test: SoundPoolAacTest SoundPoolHapticTest
Test: SoundPoolMidiTest SoundPoolOggTest
Bug: 175097719
Bug: 177287876
Change-Id: Iec777d6319d5ed76000d4c5b12336b106dacede4