FilesApp: Support available bytes in gear menu UI for DocumentsProvider

This change implements "volume-space-info" Chrome gear menu item when
there is availability of writable DocumentsProvider root(s) with given
authority.

It will enable ARC apps like Box, DropBox, etc. to show available bytes
vs. capacity bytes within the volume's gear menu.  If the root is not
writable or both available and capacity bytes columns are unbounded,
the menu item will remain hidden as before. If the root's available
bytes is provided by the DocumentsProvider but not capacity, "Available"
field in the UI will be populated but progress bar will stay hidden.

To prevent performance issues, only the requested root ID based on
authority will be queried synchronously by ContentProviderClient for
available bytes and capacity bytes since these root changes are not
always updated by the provider app via notifyChange. The counterpart
Android CL is http://ag/13490814.

BUG=953657
BUG=b:179332056
TEST=Tested manually with Android CL and verified using Box app.

Change-Id: Ifb28875fc06f3431e1dbe7d6be38ab59dc4082f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2695949
Reviewed-by: Josh Simmons <simmonsjosh@google.com>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Alan Ding <alanding@chromium.org>
Cr-Commit-Position: refs/heads/master@{#856185}