Render images in a conversation true-to-size.

Previously, we were always rendering images as squares. Instead of doing
that, we now render them as close to true-to-size as possible (within
reasonable min/max width/height boundaries).

res/values/attrs.xml

@@ -147,6 +147,10 @@
 
     <declare-styleable name="ThumbnailView">
         <attr name="backgroundColorHint" format="color" />
+        <attr name="minWidth" format="dimension" />
+        <attr name="maxWidth" format="dimension" />
+        <attr name="minHeight" format="dimension" />
+        <attr name="maxHeight" format="dimension" />
     </declare-styleable>
 
     <declare-styleable name="DeliveryStatusView">