TimedAudioTrack: Fix continuity threshold handling.

Hand merge from ics-aah

> TimedAudioTrack: Fix continuity threshold handling.
>
> Fix issues with continuity threshold handling; notably
> + If the steady-state continuity threshold is exceeded, be sure to
>   clear the on-time flag.  Failure to do this will result in the
>   system picking a new mix point which simply satisfies the
>   steady-state continuity threshold instead of the startup threshold.
>   Since we are putting a discontinuity in presentation anyway, we
>   really want to pick a perfect point, not just an OK point.
> + Tighten the steady-state continuity threshold.  It was currently set
>   to 100mSec which is enormous.  4mSec (the new setting) is much more
>   appropriate.  On systems with a VCXO (like tungsten) this should
>   never be wrong by more than a sample.  If TimedAudioTracks are ever
>   to be used on VCXO-less systems, this threshold should probably be a
>   a parameter configurable by applications on a track by track basis
>   so they can make the tradeoff between allowed error and frequency of
>   disruptive corrections.
> + Reset the on-time flag if the mixer provides no PTS during a mix
>   operation.  This makes for a convenient way for the HAL to reset
>   timed tracks when it makes changes for delay compensation across
>   multiple outputs.
>
> Change-Id: I2cb23de5a3d1f75618abc1c8ab903db883837aa8
> Signed-off-by: John Grossman <johngro@google.com>

Change-Id: Ibd28c9d290494b0b19eb01caf2d9bfdef606a9b4
Signed-off-by: John Grossman <johngro@google.com>