Initial commit for A2dp control path on linux

* Add the boilerplate code for a2dp_encoding_host.cc, which implements the
handler for linux audio server D-BUS commands.
* Add start_audio_request(), stop_audio_request(), and
get_presentation_position() which might be used by audio server.
* Open a unix domain socket when audio server requests starting an audio
straming session, and read data from it.

* Need to add callbacks and wire the code to topshim in next iteration.

Tag: #floss
Bug: 190730870
Test: compile
Change-Id: I0ca1147ec641584b2bb8a69f0d4446cd730edd61