CredentialFragment anti-pocket dialing measures.

Now it is necessary to actually press the exact "OK" or Back button to
get the dialog to dismiss. This makes pocket wiping that little bit less
likely.

Bug: 32934848
Test: manual, enter the password too many times. The dialog should appear. Attempt to dismiss it by tapping outside the dialog. This should not happen.
Test: correctness of .setCanceledOnTouchOutside is outside scope.
Change-Id: Icff8bd9068f636c0a75decb787b8a5c9161a8cbd