Improve work profile unification flow

Expose internal API to check if the user's password
will be sufficient after profile unification. Also
expose some other helper methods and refactor
DevicePolicyManagerService to unify a few similar
methods that gather admins from user and its profiles.

Bug: 148630506
Fix: 149682344
Test: atest com.android.server.locksettings
Test: atest FrameworksServicesTests:DevicePolicyManagerTest
Change-Id: Ic647c14d5bab7e7337185bc40b1368e42c65f738