Move the MIME type icon detection into the platform.

This allows the MIME type icons to be used in other system apps.
In this instance, the StorageManager app needs to show file icons
for a given file. By moving the icon mapping into a static library,
we can avoid duplicating the mapping and resources.

Bug: 29826961
Change-Id: Ic2dc7970cc5a5ca430e33425eb9f1ecbd733fe9a