Move FeatureFlagReader to sysprops

DeviceConfigs ended up being flaky and buggy. They were also eventually
reset.
SystemProperties are stable and widely used across the system.

Test: adb shell setprop persist.systemui.flag_monet 1
Test: atest FeatureFlagReaderTest
Fixes: 180559762
Change-Id: I20b1ae82ac66a2257159048ac6f9b5cd00f677c5