Add lifecycle observers for future mixin structures.

- Converted VisibilityLoggerMixin into a LifecyclerObservable so we
  don't have to call logger.onResume/onPause manually in most fragments.
- Observable will be useful when we provide logics across all
  fragment/activity, eg log lifecycle event latencies.
- Also added new tests for lifecycle component.

Bug: 30681529
Test: RunSettingsRoboTests

Change-Id: Ida39300aeb42f71b2e0bbfaebd0c51dc468cb5e8

src/com/android/settings/utils/ThreadUtils.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2016 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.utils;
+
+import android.os.Looper;
+
+public class ThreadUtils {
+    private static volatile Thread sMainThread;
+
+    /**
+     * Returns true if the current thread is the UI thread.
+     */
+    public static boolean isMainThread() {
+        if (sMainThread == null) {
+            sMainThread = Looper.getMainLooper().getThread();
+        }
+        return Thread.currentThread() == sMainThread;
+    }
+
+    /**
+     * Checks that the current thread is the UI thread. Otherwise throws an exception.
+     */
+    public static void ensureMainThread() {
+        if (!isMainThread()) {
+            throw new RuntimeException("Must be called on the UI thread");
+        }
+    }
+
+}