[Android] Put CachedFlag overrides for test in FeatureList

Before this CL, CachedFlag used ValuesReturned to override values for
testing, which is a relic of how it worked before being made into a
generic implementation (used to be each flag would re-implement the
system).

Now FeatureList's sTestFeatures holds the overrides for all types of
flags.

Bug: 40281605
Change-Id: I472a6224772a949e55e25121d321a6a18b25f685
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5916527
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1368933}