Fix dark theme and battery saver interaction

In settings we weren't properly taking into account battery saver
enabled dark theme. This CL makes it so that when battery saver
on we disable the toggle until battery saver is off since
you can't change dark theme in battery saver. Additionally it
adds a message indicating why the toggle was disabled so the
user doesn't get confused.

Test: robotests
Bug: 131913864
Change-Id: I855e429a666098c837650852d1e1355477afa13d

tests/robotests/src/com/android/settings/display/DarkUIPreferenceControllerTest.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) 2019 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.display;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import android.content.Context;
+import android.os.Handler;
+import android.os.IPowerManager;
+import android.os.PowerManager;
+
+import androidx.fragment.app.Fragment;
+import androidx.preference.PreferenceScreen;
+import androidx.preference.SwitchPreference;
+
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+import org.robolectric.RobolectricTestRunner;
+import org.robolectric.RuntimeEnvironment;
+
+@RunWith(RobolectricTestRunner.class)
+public class DarkUIPreferenceControllerTest {
+
+    private DarkUIPreferenceController mController;
+    private Context mContext;
+    @Mock
+    private Fragment mFragment;
+
+    @Before
+    public void setUp() {
+        MockitoAnnotations.initMocks(this);
+        mContext = spy(RuntimeEnvironment.application);
+        mController = spy(new DarkUIPreferenceController(mContext, "dark_ui_mode"));
+        mController.setParentFragment(mFragment);
+        mController.mPreference = new SwitchPreference(mContext);
+        mController.onStart();
+    }
+
+    @Test
+    public void batterySaverToggles_disabledStateUpdates() {
+        doReturn(true).when(mController).isPowerSaveMode();
+        mController.updateEnabledStateIfNeeded();
+        assertThat(mController.mPreference.isEnabled()).isFalse();
+
+        doReturn(false).when(mController).isPowerSaveMode();
+        mController.updateEnabledStateIfNeeded();
+        assertThat(mController.mPreference.isEnabled()).isTrue();
+
+        doReturn(true).when(mController).isPowerSaveMode();
+        mController.updateEnabledStateIfNeeded();
+        assertThat(mController.mPreference.isEnabled()).isFalse();
+    }
+}