Replace milliseconds with base::TimeDelta in ConfigurableStorageDelegate

This makes the tests more readable and will make it easier to remove
the manager unit tests' use of the real storage delegate.

Change-Id: I9b2b9504bf4bbe1a60b0526c637bdde44b8466ce
Bug: 1294782
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3448270


Reviewed-by: Nan Lin <linnan@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#968339}