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After https://crrev.com/c/2602969, |OnDeviceClosed()| might be triggered
from different threads. But in |OnDeviceClosed()|, it will access
|camera_app_devices_| and might introduce race condition.

Therefore, to fix it, we can post the task to make sure that
|OnDeviceClosed()| will be called on the same thread.

And since now we will not call |OnDeviceClosed()| directly and the mojo
error handler of a receiver set might be triggered more than once, we
should also handle it properly.

Bug: b/177191321
Test: Consecutively passes CCAUISettings.swa for 20 times

Change-Id: I93e546a98c410017f17071c97336bfe40dcc7b69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2619247
Commit-Queue: Wei Lee <wtlee@chromium.org>
Commit-Queue: Shik Chen <shik@chromium.org>
Auto-Submit: Wei Lee <wtlee@chromium.org>
Reviewed-by: Shik Chen <shik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841974}