Corrected default value assignments in Bluetooth A2DP Metrics

* In certain cases btif_a2dp_source_stop_audio_req() could be called before
  btif_a2dp_source_start_audio_req() is called, resulting in session_start_us
  to be 0. In this case, audio_duration_ms = time_get_os_boottime_us() - 0
  will be a very large number that is not the actual audio duration. This
  CL marks audio_duration_ms as -1 in the above situation so that we can
  differentiate between valid and invalid audio durations
* Set default timer and counter values to 0 when
  tx_queue_dequeue_stats.total_updates > 1
* Move update_scheduling_stats for tx_queue_enqueue_stats to
  btif_a2dp_source_audio_handle_timer as we intend to capture the time
  intervals for enqueue scheduling instead of time intervals between
  frame enqueue (i.e. one scheduling event can enqueue multiple frames)
* Use tx_queue_enqueue_stats for metrics instead of *dequeue* since only
  enqueue is triggered by timer event

Bug: 33694310
Test: Code compilation, BtFunhausMetricsTest
Change-Id: I10984920afd4d77f07a5ac75736f8dcd69b13af8
(cherry picked from commit aa79f0fb)