Return ResolvedPolicy from ResolutionMechanism::resolve

ResolutionMechanism::resolve method will return new ResolvedPolicy
class, which contains resolved policy value and the admins who has
contributed to it.

Test: atest ResolutionMechanismTest
Bug: 414733570
Flag: EXEMPT code not used yet

Change-Id: Ia13fc0774dd1235a133547baff46588f29ef7b1d