FeatureFlags Listeners:

* Convert FlagReader interface to FlagListenable.
* Move Flag reading to FeatureFlags interface.
* Remove isEnabled(id, boolean) from all interfaces; make private in implementations where used.

Bug: 209081785
Test: atest FlagManagerTest
Change-Id: I8c90a67def242b5d2fa55d5effab8270ecafabb4