audioflinger: fix audio skipping over A2DP

The maximum sleep time allowed in the mixer thread when audio tracks
are enabled but not ready for mixing is derived from the latency
reported by the output stream.
This does not work for A2DP where the latency also reflects encoding, decoding
and transfer time.

Modified activeSleepTimeUs() to take A2DP case into account.

Issue 5682206.

Change-Id: I3784ac01fb6f836b5a6ce6f764fb15347586de35