audiopolicy: Two clients can not offload concurrently

If two application are playing a compatible offload stream at the same
time.
Eg: two games playing MP3. At least one not taking the audio
focus.

The audio policy was offloading both stream resulting in audio glitches
and one stream being dropped.

Concurrent offload streams is intended to be a transitioning state when
an application wants back to back playbacks.

It should not be allowed for two different applications to stream at the
same time.

Test: Play mp3 while mp3 is already offloaded.
Bug: 34012147

Change-Id: I98a8913d6faf5092a1e43a0bdd0f1ce1482221a9
Signed-off-by: Kevin Rocard <krocard@google.com>