Add support for fallback categories.

When an app doesn't define a category, look for a fallback category
in a hard-coded list.  This change only defines a fallback for a
single package, but device-specific overlays can be used to provide
more detailed fallback lists.

The precidence order is: app manifest > installer hint > fallback

Test: builds, boots, fallback categories work
Bug: 33815939
Change-Id: I1f5ca76fb7e5743a4500c0a1230a754266f34d9e