[Passwords] Extend StoresUsedForFillingInLast28/7Days histograms

Those histograms are useful to watch over a longer period of time given
that B4P has just fully launched.

Bug: 1228318
Change-Id: I23f7775f2cf7627e19e9c7c06b3b5adf5cc218eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3021177


Auto-Submit: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Viktor Semeniuk <vsemeniuk@google.com>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#900956}