Fix elevation in double pane layout

src/main/res/layout/task_list_body_empty.xml

@@ -1,7 +1,6 @@
 <android.support.v4.widget.SwipeRefreshLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/swipe_layout_empty"
     style="@style/task_list_container"
-    android:background="?attr/asContentBackground"
     android:layout_width="fill_parent"
     android:layout_height="wrap_content"
     android:elevation="@dimen/elevation_task_list">

src/main/res/layout/task_list_body_standard.xml

@@ -1,6 +1,7 @@
 <android.support.v4.widget.SwipeRefreshLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/swipe_layout"
     style="@style/task_list_container"
+    android:background="?attr/asContentBackground"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent"
     android:elevation="@dimen/elevation_task_list">

src/main/res/layout/task_list_body_subtasks.xml

@@ -1,6 +1,7 @@
 <org.tasks.ui.DraggableSwipeRefreshLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/swipe_layout"
     style="@style/task_list_container"
+    android:background="?attr/asContentBackground"
     android:layout_width="fill_parent"
     android:layout_height="wrap_content"
     android:elevation="@dimen/elevation_task_list" >

src/main/res/values/styles.xml

@@ -53,7 +53,6 @@
         <item name="android:scrollbars">vertical</item>
         <item name="android:cacheColorHint">#00000000</item>
         <item name="android:smoothScrollbar">true</item>
-        <item name="android:background">?attr/asContentBackground</item>
     </style>
 
     <style name="TaskListView" parent="ListView" />