Floss: Wire a2dp source cleanup in topshim

Cleanup the interface by calling btav_source_interface_t::cleanup
directly.

Bug: 264745185
Test: run AVRCP/CT/PTH/BV-01-C along with the top of this series
Tag: #floss

Change-Id: I0d7bd25e938638ea79657f462826cd5b00b994e3