Add support for collecting A2DP Protobuf metrics

* Add new function metrics_a2dp_session() to
  connect A2DP metrics into Protobuf

* Add new function btif_update_a2dp_metrics()
  that is called when the metrics should be
  callected

* Add few more statistics to the BTIF media stats.

Change-Id: I5862ac82e55b3546dc1fbde0c78cb4683e40d19e