Display a generated avatar icon rather than a single default.

If the contact doesn't have an image, render a color-coded
background and the first letter of the contact's name.

1) Don't display anything during recipient resolution.

2) Display a # icon in material gray for recipients with no name.

3) Display a material group icon in material gray for groups with
   no avatar icon set.

Closes #3104

// FREEBIE

res/values/attrs.xml

@@ -113,4 +113,5 @@
     </declare-styleable>
 
     <attr name="group_members_dialog_icon" format="reference"/>
+
 </resources>