Keep FakeFeatureFlagsImpl

* Keep FakeFeatureFlagsImpl.
* Use EnableFlags and DisableFlags annotation
* Switch back from `assumeTrue` to `assert` when flag is enabled or disabled.
* Remove flag rollback test.

Bug: 342830919
Change-Id: Ic9b3438f3f0e31e64d5405ed81903b352195714a
3 files changed