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According to audio_io interface, AudioOutputStream::Stop should be
implemented in a synchronized way that no more access to source callback
will happen after Stop. However, MixerServiceWrapper::Stop is posted to
another thread, and complete asynchronizely, which potentially causes
crash, as upstream may destroyed source callback before
MixerServiceWrapper actually stops. HoweverAdd a waitable event for
MixerServiceWrapper as we did for CmaWrapper.

Bug: internal: 142355421
Test: On device and unittests.
Change-Id: Ia545596fbb80528d4f92debe3229b64fdea22667
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1866294
Commit-Queue: Kehuang Li <kehuangli@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707104}