[Android] Move legacy SharedPrefs map out of CachedFeatureFlags

The mapping is now in the CachedFlags themselves for the few that are
from 2019 or older and still use a legacy pref key.

This is a step to move CachedFlag into //base as the legacy preference
keys are Chrome-specific, so they need to remain somewhere in
//chrome as opposed to the generic code in CachedFlag and
CachedFeatureFlags that will get moved down to //base.

Disable-Rts: True
Bug: 1442347
Change-Id: I029816a098ab02fb000d8f64e3e8817598e65636
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4500337


Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1146590d}