Updated the A2DP statistics

 * Added "Frame expected" and "Frames limited" statistics in the
   bugreport about expected and limited number of frames per tick.
   Also, added corresponding "Counts (expected/limited)":

A2DP State:
  TxQueue:
  Counts (enqueue/dequeue/readbuf)                        : 386 / 386 / 768
  Last update time ago in ms (enqueue/dequeue/readbuf)    : 12644 / 12643 / 12600
  Frames expected (total/max/ave)                         : 2632 / 8 / 6
  Frames limited (total/max/ave)                          : 0 / 0 / 0
  Counts (expected/limited)                               : 382 / 0
  Frames per packet (total/max/ave)                       : 2618 / 7 / 6
  ...

 * Renamed "tx_queue_max_frames" to "tx_queue_max_frames_per_packet"

 * Fixed the computation of actual transmitted frames:
   tx_queue_total_frames, tx_queue_max_frames_per_packet

 * Computed / added buffer_overruns_max_count and
   buffer_underruns_average to the Protobuf stats.

Bug: 27500656
Change-Id: Id58cfb0b5de2296742c84aa0b743e0eed398f41f