Allow icons to be overlayed via resource config.

Since we've got got both Files and Download icons included
in the app now, there's no reasons for overlay's to replace
the Downloads icon with a Files app icon. This change will
allow them to overlay just the drawable mapping.

Bug: 30511480
Change-Id: I6e486ea81a18d0a07a8f1a8da51b1aab81ee9dd4