[Android] Move code from CachedFeatureFlags to CachedFlag

Move static methods from CachedFeatureFlags to be instance methods in
CachedFlag. This makes CachedFlag more cohesive and separates it from
CachedFeatureFlags code that concerns field trials or special-cased
flags.

This is a step to move CachedFlag to //base.

Disable-Rts: True
Bug: 1442347
Change-Id: Ic18de98a6475ada089a0d32ce7057286f7f72bf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4529355


Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1146713}