Floss: Mute false alarm UNDERFLOW after audio stop

Add |sw_audio_encoding_started| to mark the start and stop of the audio
encoding.

Bug: 300455937
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: Manually tested a2dp on ChromeOS and verified no UNDERFLOW log
Change-Id: Ia46d9868605d19c8ef996faa5ea1fd7dc103d417