Add master toggle to enable/disable screen savers

The new control toggles the Settings.Secure.SCREENSAVER_ENABLED setting,
which controls if a screen saver is enabled. Also removed the "Never"
option from the WhenToDream picker, since this is now controlled via the
toggle.

Test: locally on device
Bug: 222360260
Change-Id: Ibcf52b5e2d1ad0c295f32c4c18bfc89082d074ce

src/com/android/settings/dream/DreamMainSwitchPreferenceController.java

@@ -0,0 +1,93 @@
+/*
+ * Copyright (C) 2022 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.dream;
+
+import static androidx.lifecycle.Lifecycle.Event.ON_START;
+import static androidx.lifecycle.Lifecycle.Event.ON_STOP;
+
+import android.content.Context;
+import android.database.ContentObserver;
+import android.os.Handler;
+import android.os.Looper;
+import android.provider.Settings;
+
+import androidx.lifecycle.OnLifecycleEvent;
+
+import com.android.settings.widget.SettingsMainSwitchPreferenceController;
+import com.android.settingslib.core.lifecycle.LifecycleObserver;
+import com.android.settingslib.dream.DreamBackend;
+
+/**
+ * Preference controller for switching dreams on/off.
+ */
+public class DreamMainSwitchPreferenceController extends
+        SettingsMainSwitchPreferenceController implements LifecycleObserver {
+    static final String MAIN_SWITCH_PREF_KEY = "dream_main_settings_switch";
+    private final DreamBackend mBackend;
+
+    private final ContentObserver mObserver = new ContentObserver(
+            new Handler(Looper.getMainLooper())) {
+        @Override
+        public void onChange(boolean selfChange) {
+            updateState(mSwitchPreference);
+        }
+    };
+
+    public DreamMainSwitchPreferenceController(Context context, String key) {
+        super(context, key);
+        mBackend = DreamBackend.getInstance(context);
+    }
+
+    @Override
+    public int getAvailabilityStatus() {
+        return AVAILABLE;
+    }
+
+    @Override
+    public boolean isChecked() {
+        return mBackend.isEnabled();
+    }
+
+    @Override
+    public boolean setChecked(boolean isChecked) {
+        mBackend.setEnabled(isChecked);
+        return true;
+    }
+
+    @Override
+    public boolean isSliceable() {
+        return false;
+    }
+
+    @Override
+    public int getSliceHighlightMenuRes() {
+        // not needed since it's not sliceable
+        return NO_RES;
+    }
+
+    @OnLifecycleEvent(ON_START)
+    void onStart() {
+        mContext.getContentResolver().registerContentObserver(
+                Settings.Secure.getUriFor(Settings.Secure.SCREENSAVER_ENABLED),
+                /* notifyForDescendants= */ false, mObserver);
+    }
+
+    @OnLifecycleEvent(ON_STOP)
+    void onStop() {
+        mContext.getContentResolver().unregisterContentObserver(mObserver);
+    }
+}