Unit test for btif avrcp audio track

New unit-tests are added to validate gain is set appropriately for the
audio track buffer. It also validates the transcoding of the audio
buffer does use the appropriate gain value.

Bug: 278602076
Tag: #stability
Test: atest net_test_btif_avrcp_audio_track --iterations 20
Change-Id: I9656d77d554ebef4b189599ca9ad5e8ee9746f53