DPMS: Passing in the right user ID for password checking

Rather than pass in the user ID and parent flag, resolve the right user
for which the password requirements need to be checked/retrieved.

This refactoring should introduce no functional changes.

Bug: 165573442
Test: ates com.android.cts.devicepolicy.MixedDeviceOwnerTest#testResetPasswordWithToken com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testResetPasswordWithToken
Change-Id: I97ca0d40a01673939e64c23f357fc38ca5427a8f