Connect FeatureFlags with FeatureFlagsService.

With this change, android.flags.FeatureFlags' internals are
filled out, connecting with with FeatureFlagsService to create
a working implementation.

Bug: 279054964
Test: atest FrameworkksCoreTests:android.flags
Test: manually run adb shell commands.
Change-Id: I37b641a5571cf57dd9b5f1373549931fdd9bb5a3
12 files changed