Add topshim for A2DP source interface

- btav_shim to access the btav_source_interface_t
- a2dp.rs to call methods on a2dp source interface and
  register callbacks through the cb_variant framework.

Bug: 190676649
Tag: #floss
Test: Build floss

Change-Id: I3d71ec152b8e33ba9b398b75070446b5a7fd3db7