arc: Handle action OPEN_MEDIA_STORE_FILES

The new action is dealt with in the same way as OPEN_DOCUMENT except
that the file selector (Files app) only shows volumes that are indexed
in Android's MediaStore (namely "My files" and removable media).
Corresponding Android side change is http://ag/18555373.

Bug: b:234990050
Test: unit_tests --gtest-filter="ArcSelectFilesHandlerTest.*"
Test: tast run <DUT> arc.VolumeFilter* (crrev.com/c/3659119)
Change-Id: I806ad3c20892c70f8ba9c297429b90ff1a987498
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3657346
Commit-Queue: Youkichi Hosoi <youkichihosoi@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1011320}