Create unit test framework for audio_a2dp_hw

Create unit test framework for audio_a2dp_hw and implement a unit test
for function audio_a2dp_hw_stream_compute_buffer_size()

Also:
 * Remove obsolete check for mixer unit size inside out_get_buffer_size()
 * Reorganize the file structure inside directory audio_a2dp_hw/ :
   - Moved the source code files *.cc to audio_a2dp_hw/src/
   - Moved the header file(s) *.h to audio_a2dp_hw/include/
   - The unit test file(s) are stored in audio_a2dp_hw/test/

Bug: 35849921
Test: This is the unit test. Code compilation and unit test succeed
Change-Id: I3dcc508cb6799462b7103cb2ee452b4399843899