AudioFlinger: fix timed audio

(cherry picked from commit e20ac92c564a2f4e8123885807abdf0a78de0dd7)

> AudioFlinger: fix timed audio
>
> Addresses Bug 6900517.
>
> Finish up support for timed audio in the new FastMixer world.  Pay special
> attention to remaining lock-less and voluntary yield free on the FastMixer
> thread.  This fixes audio playback for Q on JB.
>
> Change-Id: Iaf815e58a1b1d0a0190051794bec8dc5c9231785
> Signed-off-by: John Grossman <johngro@google.com>

Change-Id: I9bd687acc345a05867af48e71116690fdb0ce1b5
Signed-off-by: John Grossman <johngro@google.com>