Add DeviceConfig feature flag type to SystemUI

Adds support of DeviceConfig feature flags.
DeviceConfig flags are used by Phenotype
A/B testing framework.

Bug: 218314434
Test: change phenotype flag =>
 check that FeatureFlags returns correct value
Test: atest com.android.systemui.flags.FeatureFlagsDebugTest
Test: atest com.android.systemui.flags.FeatureFlagsReleaseTest
Change-Id: I7dd9e47280ba56b7f9ea552c0d44d3e36fc6b041