Apply gain to the audio track stream

Problem: While bluetooth is streaming media (with appropriate audio
focus), any other media/audio app can acquire transient audio focus
i.e. AUDIOFOCUS_GAIN_TRANSIENT and AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK
This results in bluetooth process to receive the respective LOSS
events. When it receives the AUDIOFOCUS_LOSS_TRANSIENT it sends a
pause command to the remote device. In addition, it also mutes the
stream in case the remote device rejects the pause command to ensure
no sound is made by the bluetooth process. The logic to set the gain
was broken post the lib AAudio migration resulting in bluetooth
process making additional sound on top of the other audio app.
Similarly, when other media/audio app had requested for gain_may_duck
focus, bluetooth process wasn't ducking the media sound resulting in
overlap of both the audio.

Solution: Updated the business logic to update the stream buffer
based on the gain value set by the bluetooth process. On different
transient audio focus events, different gain (in range of [0.0f,1.0f])
is set by the handler. On focus gain event, it sets back the gain to
1.0f.

Bug:254724838
Tag:#stability
Test: Tested by setting the different gain value in the code and
validating on device.

Change-Id: Iafedf759949df6e54cad7f21d64dc247712fd380