Messaging: Style the attachments properly

* With dark theme, they suddenly don't stand out from the background
  anymore
* Give them more rounding but not as much as the chat bubbles

Change-Id: I32c1036bda348e0c58e1cb37edbe9c7fc72309e7

res/layout/attachment_chooser_audio.xml

@@ -20,7 +20,7 @@
     android:id="@+id/audio_attachment_background"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:background="@color/attachment_chooser_audio_background">
+    android:background="@drawable/attachment_audio_preview_background">
 
     <com.android.messaging.ui.AudioAttachmentView
         android:id="@+id/audio_attachment_view"

res/layout/attachment_chooser_image.xml

@@ -23,4 +23,5 @@
     android:minWidth="@dimen/single_attachment_min_dimen"
     android:minHeight="@dimen/single_attachment_min_dimen"
     android:scaleType="centerCrop"
-    android:adjustViewBounds="true" />
+    android:adjustViewBounds="true"
+    app:cornerRadius="@dimen/attachment_rounded_corner_radius"/>

res/layout/attachment_chooser_vcard.xml

@@ -17,7 +17,7 @@
 <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:background="@color/attachment_chooser_vcard_background">
+    android:background="@drawable/attachment_vcard_preview_background">
 
     <com.android.messaging.ui.PersonItemView
         android:id="@+id/vcard_attachment_view"