[FilesRecentFilter] Documents filter for ARC

Support documents_root in RecentArcMediaSource:
* Add documents_root to the arc_documents_provider.
* Register the new root in the volume_manager.
* Refactor: remove all hard-code root string and use constants from
arc_media_view_util.

Note: this feature only works with ARCVM (android 11), because that's
when Android's MediaDocumentsProvider.queryRecentDocuments()
introduces documents_root support. For ARC++ containers, no files will
return for documents_root.

Demo: http://shortn/_ezNIvqcryC
Demo for Android 9: http://shortn/_4Xooka3i1J (image/video filters still
work, but the document file .doc is not showing in the All/Documents
filter)

folders and restart chromebook, then check the newly created files
should appear in Recents document filter

Bug: 881673
Test: unit_tests --gtest_filter="*RecentArcMediaSourceTest*"
Test: unit_tests --gtest_filter="*VolumeManagetTest*"
Test: unit_tests --gtest_filter="*FileManagerPathUtilTest*"
Test: manually create doc files (e.g. txt/pdf/doc) in the "Play files"
Change-Id: I98679e48c59560799deb05bbf9351885e9157079
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3596676


Commit-Queue: Wenbo Jie <wenbojie@chromium.org>
Reviewed-by: Austin Tankiang <austinct@chromium.org>
Reviewed-by: David Jacobo <djacobo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999778}