A2DP Offload: Log audio duration and codec metrics

* Add A2dpOffloadAudioStats class to store metrics for A2DP offload
* Add logging statements to log A2DP start/suspend/stop events when A2DP
  offload is enabled
* Separate software encoding logging from hardware encoding logging

Bug: 109953464
Test: make, run A2DP in offload mode and dump metrics
Change-Id: I1a19c64c6b1ee530e5927dd3d13e31b00729850b