Added Start/End Session steps when A2DP codec is changed

If the A2DP codec is changed for the active device, then
the current session is restarted by calling End/Start Session.

Also:
 * Updated btav_a2dp_codec_config_t string representation to
   include the codec priority.
 * Fixed A2dpCodecConfig::setCodecPriority() and setDefaultCodecPriority()
   so the codec priority for the current codec_config_ is updated.
 * Fixed A2dpCodecs::setCodecUserConfig() so the restart_input flag
   is set as appropriate.
 * Use btav_a2dp_codec_config_t::ToString() as appropritate to print
   the codec config instead of explicitly printing each field.

Bug: 74988739
Test: Manual: Connect two headsets, change codecs, switch active device.
Change-Id: I6652168f3c48e5b431e00aa8f554929afbdcdbcf