pt 1 in reviving the android build (copied from pm 28a1, wish me luck)

mobile/android/base/java/org/mozilla/gecko/ScreenManagerHelper.java

@@ -0,0 +1,43 @@
+/* -*- Mode: Java; c-basic-offset: 4; tab-width: 20; indent-tabs-mode: nil; -*-
+ * vim: ts=4 sw=4 expandtab:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.gecko;
+
+import org.mozilla.gecko.annotation.WrapForJNI;
+
+class ScreenManagerHelper {
+
+    /**
+     * The following display types use the same definition in nsIScreen.idl
+     */
+    final static int DISPLAY_PRIMARY  = 0; // primary screen
+    final static int DISPLAY_EXTERNAL = 1; // wired displays, such as HDMI, DisplayPort, etc.
+    final static int DISPLAY_VIRTUAL  = 2; // wireless displays, such as Chromecast, WiFi-Display, etc.
+
+    /**
+     * Add a new nsScreen when a new display in Android is available.
+     *
+     * @param displayType the display type of the nsScreen would be added
+     * @param width       the width of the new nsScreen
+     * @param height      the height of the new nsScreen
+     * @param density     the density of the new nsScreen
+     *
+     * @return            return the ID of the added nsScreen
+     */
+    @WrapForJNI
+    public native static int addDisplay(int displayType,
+                                        int width,
+                                        int height,
+                                        float density);
+
+    /**
+     * Remove the nsScreen by the specific screen ID.
+     *
+     * @param screenId    the ID of the screen would be removed.
+     */
+    @WrapForJNI
+    public native static void removeDisplay(int screenId);
+}