Add a sample multi-client IME

This CL adds a sample multi-client IME.  Ahthough there are still many
limitations and missing features in this sample IME, this CL
demonstrates how a multi-client IME can be implemented.

Fix: 115784148
Test: prebuilts/checkstyle/checkstyle.py -f \
      development/samples/MultiClientInputMethod/
Test: Manually verified as follows:
  1. make -j MultiClientInputMethod
  2. adb install -r $OUT/system/priv-app/MultiClientInputMethod/MultiClientInputMethod.apk
  3. adb root
  4. adb shell setprop persist.debug.multi_client_ime \
       com.example.android.multiclientinputmethod/.MultiClientInputMethod
  5. adb reboot
  6. Try multiple text input scenario
Change-Id: Ide43e16448fa00355a2c08bc45ae94d98553da50

samples/MultiClientInputMethod/res/layout/input.xml

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+     Copyright (C) 2018 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.
+-->
+
+
+<android.inputmethodservice.KeyboardView
+        xmlns:android="http://schemas.android.com/apk/res/android"
+        android:id="@+id/keyboard"
+        android:layout_alignParentBottom="true"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        />