Improve work profile unification flow

When unifying work profile challenge, keep the device lock
as long as it will still meet password requirement after unification.
If not, prompt the user to set a new device lock and only unify
work challenge after a compliant device lock is set.

Bug: 148630506
Fix: 149682344
Test: make RunSettingsRoboTests
ROBOTEST_FILTER='ChooseLockGenericTest|ChooseLockPasswordTest|ChooseLockPatternTest|LockUnificationPreferenceControllerTest'

Change-Id: I99cde2650902927f6a4cc7c0cc7c6016e0dc283f