[PEPC] Revise heuristic grant expiration to use a decay model

This CL extended the expiration time to 28 days. When an auto-grant
expires, the temporary grant count no longer resets to zero. Instead,
it "decays" to just below the grant threshold.

This allows a user who has previously established an auto-grant to
re-establish it with only a single new temporary grant, reducing user
friction while still ensuring that inactive grants eventually expire.
If a grant count below the threshold expires, it resets to zero as
before.

The `RecordTemporaryGrant` method has also been revised to record both
the grant count and timestamp.

Bug: 448564901
Change-Id: I1d3675a7e2355678581ac864e4756031e1bdae0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7026231
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Thomas Nguyen <tungnh@chromium.org>
Reviewed-by: Andy Paicu <andypaicu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1528891}