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>