add "elevation" window overlay for android 4.x too

// FREEBIE

res/drawable-v16/compat_actionbar_shadow_background.xml

@@ -3,7 +3,13 @@
 
     <item>
         <shape android:shape="rectangle">
-            <solid android:color="@android:color/transparent" />
+            <solid android:color="@color/conversation_item_sent_shadow_light" />
+        </shape>
+    </item>
+
+    <item android:bottom="1dp">
+        <shape xmlns:android="http://schemas.android.com/apk/res/android">
+            <solid android:color="@color/conversation_list_divider_light" />
         </shape>
     </item>
 

res/drawable-v21/compat_actionbar_shadow_background.xml

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<color xmlns:android="http://schemas.android.com/apk/res/android" android:color="@android:color/transparent">
+</color>