Metrics: Log A2DP playback related atoms to StatsLog

* BluetoothA2dpAudioOverrunReported
  - Logs when A2DP failed send encoded data to the remote device fast
    enough such that the transmit buffer queue is full and we have to
    drop data
* BluetoothA2dpAudioUnderrunReported
  - Logs when A2DP failed to read from PCM source
* BluetoothDeviceRssiReported
  - Logged when remote device's RSSI level is reported by the controller
* BluetoothDeviceFailedContactCounterReported
  - Logged when ACL transmit queue was flushed by the controller
  - This is normally disabled unless we set automatic flush timeout was
    set to a non-zero value
  - However, when this event is reported, the connection quality must
    be very bad
* BluetoothDeviceTxPowerLevelReported
  - Logged when transmit power level is reported for a particular
    connection

Note on num_dropped_nanoseconds entry:
  - This is the duration of audio dropped and is calculated by
    multiplying the number of encoding actions with length of encoding
    interval

Bug: 112969790
Test: make, testdrive with statsd
Change-Id: Ie6aa5ee71dea213e452497dd181fee91eea8f7b4