Added metrics for A2DP transmission queue

Collect and display A2DP metrics related to the A2DP transmission queue.
The result can be displayed by "adb shell dumpsys bluetooth_manager":

A2DP State:

  TxQueue:
    Counts (enqueue/dequeue/readbuf)                        : 1767 / 1767 / 3504
    Last update time ago in ms (enqueue/dequeue/readbuf)    : 3 / 3 / 3
    Frames per packet (total/max/ave)                       : 12210 / 11 / 6
    Counts (flushed/dropped/dropouts)                       : 0 / 0 / 0
    Last update time ago in ms (flushed/dropped)            : 0 / 0
    Counts (underflow/underrun)                             : 0 / 0
    Bytes (underflow/underrun)                              : 0 / 0
    Last update time ago in ms (underflow/underrun)         : 0 / 0
    Enqueue deviation counts (overdue/premature)            : 893 / 871
    Enqueue overdue scheduling time in ms (total/max/ave)   : 242 / 12 / 0
    Enqueue premature scheduling time in ms (total/max/ave) : 842 / 20 / 0
    Dequeue deviation counts (overdue/premature)            : 869 / 897
    Dequeue overdue scheduling time in ms (total/max/ave)   : 604 / 11 / 0
    Dequeue premature scheduling time in ms (total/max/ave) : 1204 / 19 / 1

Change-Id: I75c8f24a26cf9ce7a4164a142d4ac286ff17f322