Fix A2DP metrics session duration

* Fixed A2DP duration counting. It is now counting from music play start
  to music play end.
* Start logging a2dp connection as Bluetooth sessions. Currently, only
  A2DP connections are logged. Thus the bluetooth session length will be
  the total connection length and the length within A2DP session message
  will be the audio connection length.
* Add a audio_duration_millis field in A2DPSession to record audio duration
* Add bonded memory constraint for metrics entries
* Use a builder mechanism to only build metrics upon dumping
* Refactor metrics module into BluetoothMetricsLogger class
* Created unit test for BluetoothMetricsLogger

Bug: 33694310
Test: Code compilation, Unit test, BtFunhausMetricsTest
Change-Id: Iea2a997c4ea074687a5d50860e9229f0e1b82659