Guard double-showing of user credentials dialog

Only create & show the dialogfragment if it does not already exist.

Bug: 28058930
Change-Id: Ia48cc8a5fd60a12c3beb6c9263036a449cf8d67c

src/com/android/settings/UserCredentialsSettings.java

@@ -104,11 +104,13 @@ public class UserCredentialsSettings extends OptionsMenuFragment implements OnIt
             final Bundle args = new Bundle();
             args.putParcelable(ARG_CREDENTIAL, item);
 
-            final CredentialDialogFragment frag = new CredentialDialogFragment();
+            if (target.getFragmentManager().findFragmentByTag(TAG) == null) {
+                final DialogFragment frag = new CredentialDialogFragment();
                 frag.setTargetFragment(target, /* requestCode */ -1);
                 frag.setArguments(args);
                 frag.show(target.getFragmentManager(), TAG);
             }
+        }
 
         @Override
         public Dialog onCreateDialog(Bundle savedInstanceState) {