audiopolicy: Two session can not offload concurrently

Previously the patch I98a8913d6faf5092a1e43a0bdd0f1ce1482221a9
forbade two tracks from different application to be offloaded
concurrently.

Concurrent offload is only needed for back to back playback...
Detect this use case by checking the session id instead of the
PID.

Test: call & play music & assistant
Test: run new CTS testConcurentPlayAudio
Bug: 35841692
Change-Id: I2c09c54adedf12644205bef115f1a2e16b05de91
Signed-off-by: Kevin Rocard <krocard@google.com>