[FilesBulkPinning] Add daily DSS availability percentage metric

The current metric DSSAvailabilityPercentage only gets emitted once a
change is detected. This can lead to lots of metrics coming in whilst
the initial sync is happening but only one being emitted when syncing is
stabilized.

It seems this skews the metrics slightly as a user might go from 0%
availabile to 100% and stay at 100% from then on, yet we need to make
sure the 100% is representative as the final figure, not the increase
from 0% to 100%.

This new metric will get emitted after a daily period has elapsed and
will represent a more point in time capture. So in the previous example
the 0% to 100% increase will likely get ignored as this will happen in
the first day and the final 100% is more indicative.

Fixed: b/300534960
Change-Id: I2997a4ac056117c866ddeb4972abedb620d156c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4862928
Reviewed-by: François Degros <fdegros@chromium.org>
Reviewed-by: Nigel Tao <nigeltao@chromium.org>
Commit-Queue: Ben Reich <benreich@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1200047}