Source slot IDs and capacities from config XML.

Slot IDs and capacities are defined in XML now. This allowes OEMs with
greater flexibility to override how many slots and how many affordances
can be in slot on the lock screen.

Our internal, shared library, still makes hard-coded assumptions about
there only being the expected number and IDs for slots, which is okay
since that code won't necessarily be used by OEMs.

Fix: 256195304
Test: Unit tests expanded

Change-Id: Ic2752e80cdda7495d3f03085597f3b995f461bda