FeatureManager allows the logic to be centralized and allows clients to
ask about its state. For instance, SkinVector will make use of it in
I70277c1082a504fbd5f6023e9873e8071de7e35d.
Also:
* Adds WvuiSearchTreatmentRequirementTest to test A/B logic
WvuiSearchTreatmentRequirement/Test logic are adapted from
I878239a85ffbecb5e78d73aed5568c56dbd7d659.
Bug: T270202
Change-Id: Ia02349a7b41c7caf26fbd728e0be7d47488b97e5