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When running with DriveFS enabled for the first time, recover any files
in the local cache marked as dirty to a folder in Downloads, similar to
the existing process for recovering from database errors, and clean up
any old Drive client metadata. The existing Drive client code does not
expect dirty files to be removed from its cache so clearing its metadata
is necessary to limit confusion when switching back from DriveFS.

Reset the DriveFS migration pref when running without DriveFS; each
first DriveFS after running without DriveFS should migrate pinned files
and recover any unsynced files. This should avoid losing data if a user
toggles between DriveFS and not multiple times with unsynced files.

Move Drive and DriveFS data to separate subdirectories in files app
browsertests to avoid interference, in particular in DriveFS migration
tests.

Bug: 883242
Tbr: slangley@chromium.org
Change-Id: I20199ffd04ad7f63cb77ac9a53e49ba1bf830502
Reviewed-on: https://chromium-review.googlesource.com/1242783
Commit-Queue: Sam McNally <sammc@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Sergei Datsenko <dats@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594585}