Add drop shadow under attachment drawer.

res/drawable/attachment_selector_shadow.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle">
+
+    <gradient
+        android:startColor="@color/transparent_black_30"
+        android:endColor="@color/transparent_black"
+        android:angle="90"/>
+
+</shape>

res/layout/attachment_type_selector.xml

@@ -1,7 +1,16 @@
 <?xml version="1.0" encoding="utf-8"?>
+
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
               xmlns:app="http://schemas.android.com/apk/res-auto"
               android:orientation="vertical"
+              android:layout_width="match_parent"
+              android:layout_height="wrap_content">
+
+    <View android:layout_width="match_parent"
+          android:layout_height="4dp"
+          android:background="@drawable/attachment_selector_shadow"/>
+
+    <LinearLayout android:orientation="vertical"
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:background="?attachment_type_selector_background">
@@ -221,4 +230,6 @@
 
         </LinearLayout>
 
+    </LinearLayout>
+
 </LinearLayout>

res/values/colors.xml

@@ -57,6 +57,7 @@
 
     <color name="sticker_selected_color">#99ffffff</color>
     <color name="transparent">#00FFFFFF</color>
+    <color name="transparent_black">#00000000</color>
 
     <color name="MediaOverview_Media_selected_overlay">#88000000</color>