Support multi-select in storage UI.

When caller has specified that multiple documents are okay, enable
multi-select action mode.  Currently only allows document selection,
not directories.  Returns multiple documents through ClipData.

Fix bug where GridView was stuck with 2 columns on tablets.

Change-Id: Id49b29a86330639b56fa116d37e7f0d874980c5b
diff --git a/res/drawable/item_background.xml b/res/drawable/item_background.xml
new file mode 100644
index 0000000..4fb32fc
--- /dev/null
+++ b/res/drawable/item_background.xml
@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (C) 2013 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.
+-->
+
+<selector xmlns:android="http://schemas.android.com/apk/res/android">
+    <item android:state_activated="true"                               android:state_pressed="true" android:drawable="@*android:drawable/list_activated_holo" />
+    <item android:state_activated="true"                                                            android:drawable="@*android:drawable/list_activated_holo" />
+    <item android:state_focused="true"   android:state_enabled="false" android:state_pressed="true" android:drawable="@*android:drawable/list_selector_disabled_holo_light" />
+    <item android:state_focused="true"   android:state_enabled="false"                              android:drawable="@*android:drawable/list_selector_disabled_holo_light" />
+    <item android:state_focused="true"                                 android:state_pressed="true" android:drawable="@*android:drawable/list_selector_background_transition_holo_light" />
+    <item android:state_focused="false"                                android:state_pressed="true" android:drawable="@*android:drawable/list_selector_background_transition_holo_light" />
+    <item android:state_focused="true"                                                              android:drawable="@*android:drawable/list_focused_holo" />
+    <item                                                                                           android:drawable="@android:color/transparent" />
+</selector>