Don't show multiple wipe warning dialogs

Stacking them on top of each other like that does not count as defence
in depth and we should not do it.

Bug: 32934848
Test: make RunSettingsRoboTests # added one specifically for this
Change-Id: I9490652c05a630e7c3f9164a9144fadfc0f41ea1

tests/robotests/src/com/android/settings/ConfirmCredentialTest.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License
+ */
+
+package com.android.settings;
+
+import static com.android.settings.ConfirmDeviceCredentialBaseFragment.LastTryDialog;
+import static com.google.common.truth.Truth.assertThat;
+
+import android.R;
+import android.app.Activity;
+import android.app.Fragment;
+import android.app.FragmentManager;
+
+import com.android.settings.testutils.shadow.SettingsShadowResources;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.Robolectric;
+import org.robolectric.annotation.Config;
+
+@RunWith(SettingsRobolectricTestRunner.class)
+@Config(
+        manifest = TestConfig.MANIFEST_PATH,
+        sdk = TestConfig.SDK_VERSION,
+        shadows = {
+                SettingsShadowResources.class,
+                SettingsShadowResources.SettingsShadowTheme.class,
+        })
+public class ConfirmCredentialTest {
+    @Test
+    public void testLastTryDialogShownExactlyOnce() {
+        FragmentManager fm = Robolectric.buildActivity(Activity.class).get().getFragmentManager();
+
+        // Launch only one instance at a time.
+        assertThat(LastTryDialog.show(fm, "title", R.string.yes, R.string.ok, false)).isTrue();
+        assertThat(LastTryDialog.show(fm, "title", R.string.yes, R.string.ok, false)).isFalse();
+
+        // After cancelling, the dialog should be re-shown when asked for.
+        LastTryDialog.hide(fm);
+        assertThat(LastTryDialog.show(fm, "title", R.string.yes, R.string.ok, false)).isTrue();
+    }
+}