Converts FeatureFlags to use overlayable resource flags

Updates the FeatureFlags wrapper to be backed by resource booleans.
These flags can be modified via RRO to create different build
configurations.

These flags can be locally overridden on all configurations where
are_flags_overrideable is true. It's intended that this value is
true for development builds but not production ones.

Test: atest SystemUITests:FeatureFlagReaderTest
Bug: 177954419
Change-Id: I847fa2659c0ca000431681f24f460269efbd49af