Add ability to read flag values to Flag Library.

The FlagManager now partially implements the FlagReader interface.

The FeatureFlagManager, part of SystemUI proper, now uses the
FlagManager to read flag values, helping to ensure consistency between
the two separate systems.

Bug: 203548827
Test: manual
Change-Id: Ia3b103f783a846cfe9437cd8e6b987962cebabde
10 files changed