[power bookmarks] React to background changes in subscriptions

The subscription state can change while the bottomsheet is displayed.
This could leave the bottomsheet out of date and potentially make bad
requests.

Bug: 1354230
Change-Id: I1147d278cb807f9e2b520faabb00487086e2249b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3829038


Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1041163}