TimedAudio: Track of the number of pending frames.

This is a manual merge from ics-aah

> TimedAudio: Track of the number of pending frames.
>
> Keep track of the number of frames pending in the timed audio queue so
> we can implement framesReady in O(1) time instead of O(N).  This
> change partially addresses bug 6020970; the bug will be completely
> addressed once this change has been up-integrated into master.
>
> Change-Id: I599eb15ea1f6d715b97b30e65214fb6fadd169df
> Signed-off-by: John Grossman <johngro@google.com>

Change-Id: I6cbbbc3afc8efd066fe94865326ede0c6b3db2bd
Signed-off-by: John Grossman <johngro@google.com>