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>