topshim: a2dp - Expose SetAudioConfig method to dbus

- Rename Start/StopSession to Start/StopAudioRequest
- Expose SetAudioConfig for audio server
- Validate sample rate, bits per sample, and channel mode
in SetAudioConfig method

Bug: 189497374
Tag: #floss
Test: Manual test with audio server changes calling
SetAudioConfig with 44100, 16bits, stereo

Change-Id: Ie271bfd1e05f295198a969a69543dc90f739cddd