Remove FeatureFlagReader. Allow resource overlays.

Removes the FeatureFlagReader which some "flags" still used because
they were read from resources instead of the flaggin system.

Flags can now specify a resource id where their default value can be
read from (if there is a resource definition for it).

Bug: 203548827
Test: atest SystemUITests && manual
Change-Id: Ifb895fd6c5b40d4adfe2997f4ce1f4d7d2145183