Refactors keyboard related code so mash can use a keyboard

ui/keyboard/keyboard.mojom

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module keyboard.mojom;
+
+import "ui/mojo/geometry/geometry.mojom";
+
+interface KeyboardObserver {
+  // Sent any time state changes in the keyboard.
+  OnKeyboardStateChanged(bool is_enabled,
+                         bool is_visible,
+                         uint64 display_id,
+                         mojo.Rect bounds);
+};
+
+interface Keyboard {
+  // TODO(sky): needs display id.
+  Show();
+  Hide();
+
+  // Adds an observer. OnKeyboardStateChanged() is immediately called to give
+  // the initial state.
+  AddObserver(KeyboardObserver observer);
+};