Start using ResolutionMechanism#isPolicyApplied

Added PolicyState#isPolicyApplied that delegates to the corresponding
ResolutionMechanism.

Extracted legacy code for checking whether the policy has been applied
to a new method inside DevicePolicyEngine.
Depending on a feature flag, the method uses either the new
implementation PolicyState#isPolicyApplied, either the legacy one.

Bug: 285532044
Test: atest DevicePolicyEngineTest
Flag: android.app.admin.flags.remove_hack_in_policy_engine
Change-Id: I566484569a00e669f675f5e59521391ec517b332