Switch to work challenge if MP calls resetPassword

If the profile owner wants to set a lock screen for a profile which they
created, we should let them. This will cancel any lock screen
unification that has been set up.

Attempting to clear the password will continue to throw
SecurityException if called from a managed profile.

Bug: 26682008
Change-Id: Ia09aef879a21c074ccb517905e43f62696837998