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Currently, Removable Media is watched at the top-level directory under
/media/removable. Unfortunately, this only works if the removable media
was attached before the inotify is attached (since inotify does not
detect mount event).

This changes listen to the mount events from cros-disks through
ArcVolumeManager (to synchronize with Android volume mounting) and
create a FilePathWatcher per removable media.

BUG=b:150883493
TEST=Mount invisible removable media, the media changes are indexed.
TEST=Reboot DUT with non-visible removable media attached, still indexed.
TEST=Change the removable media to be visible, still indexed.
TEST=Reboot DUT with visible media attached, still indexed
TEST=Change the removable media to be invisible, still indexed.

Change-Id: Ia02012676681e025431d74f2c3f9b951a77ca783
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362376
Auto-Submit: Risan <risan@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Risan <risan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806716}