Release React Native for Android

This is an early release and there are several things that are known
not to work if you're porting your iOS app to Android.

See the Known Issues guide on the website.

We will work with the community to reach platform parity with iOS.

ReactAndroid/src/main/java/com/facebook/react/bridge/UiThreadUtil.java

@@ -0,0 +1,56 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.bridge;
+
+import javax.annotation.Nullable;
+
+import android.os.Handler;
+import android.os.Looper;
+
+/**
+ * Utility for interacting with the UI thread.
+ */
+public class UiThreadUtil {
+
+  @Nullable private static Handler sMainHandler;
+
+  /**
+   * @return whether the current thread is the UI thread.
+   */
+  public static boolean isOnUiThread() {
+    return Looper.getMainLooper().getThread() == Thread.currentThread();
+  }
+
+  /**
+   * Throws a {@link AssertionException} if the current thread is not the UI thread.
+   */
+  public static void assertOnUiThread() {
+    SoftAssertions.assertCondition(isOnUiThread(), "Expected to run on UI thread!");
+  }
+
+  /**
+   * Throws a {@link AssertionException} if the current thread is the UI thread.
+   */
+  public static void assertNotOnUiThread() {
+    SoftAssertions.assertCondition(!isOnUiThread(), "Expected not to run on UI thread!");
+  }
+
+  /**
+   * Runs the given Runnable on the UI thread.
+   */
+  public static void runOnUiThread(Runnable runnable) {
+    synchronized (UiThreadUtil.class) {
+      if (sMainHandler == null) {
+        sMainHandler = new Handler(Looper.getMainLooper());
+      }
+    }
+    sMainHandler.post(runnable);
+  }
+}