Generate FakeFeatureFlagsImpl in test mode

Before FeatureFlagsImpl will be used as a fake for testing. This change
adds new class FakeFeatureFlagsImpl. The FeatureFlagsImpl will keep the
same as production. FakeFeatureFlagsImpl can be used as fake for
testing.

FakeFeatureFlagsImpl, and FeatureFlagsImpl will be generated in both
test and prod mode. In test mode FeatureFlagsImpl will just be a stub,
and in prod mode FakeFeatureFlagsImpl will just be a stub.

Bug: 280833463
Test: atest aconfig.test
Change-Id: I11c1e716a9ea00d55600e5e9d5fb6442420762e6