Allow Files app to be enabled via overlay.

Actual changes to Andromeda overlay are in: http://ag/733602/

Change-Id: Iad5a4cb2d087d574613ba6e9c9c9c69bd8611e8c

packages/DocumentsUI/AndroidManifest.xml

@@ -51,7 +51,7 @@
             android:name=".StandaloneActivity"
             android:theme="@style/StandaloneTheme"
             android:icon="@drawable/ic_doc_text"
-            android:enabled="false">
+            android:enabled="@bool/productivity_device">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />

packages/DocumentsUI/res/values/config.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (C) 2015 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.
+-->
+
+<resources>
+    <bool name="productivity_device">true</bool>
+</resources>