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Improve the initial implementation by adding proper pause/resume/seek support.

The AudioDecoderForMixer will notify the AvSync of these actions. The
AvSync will turn on/off its AV monitoring as needed, and will reset its
linear regressions as needed.

BUG=internal 73746352
TEST=On device, pause/play work very well now, without any
noticeable jitter by myself at all. Correction logs indicate that we
will do a hard correction after Seek and usually a soft correction after
resume, but there is no visible jitter to me at all

Change-Id: I21376e41eee15161098578208625974ff0385cf8
Reviewed-on: https://chromium-review.googlesource.com/947720
Commit-Queue: Mina Almasry <almasrymina@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541742}