Introduce DISALLOW_UNIFIED_PASSWORD.

When DISALLOW_UNIFIED_PASSWORD is enforced by managed profile
owner, the user is disallowed to user single lock for both primary
user and the profile.

DMP.isUsingUnifiedPassword() can be called by DPC to check if
this restriction is obeyed.

Test: make cts-verifier
Test: cts-tradefed run cts -m CtsDevicePolicyManagerTestCases -t
  com.android.cts.devicepolicy.ManagedProfileTest#testIsUsingUnifiedPassword
Test: cts-tradefed run cts -m CtsAdminTestCases -t
  android.admin.cts.DevicePolicyManagerTest#testIsUsingUnifiedPassword_failIfNotProfileOwner
Bug: 63909482
Change-Id: Ib758e32d4bf4012d805185bce874f481e17576ba