Teach DCS about cluster packages.

For the time being, DCS is going to still be doing heavy lifting for
some install tasks, so it need to know how to handle both monolithic
and cluster packages.  This change is mostly plumbing work to
eventually handle any various splits APKs that we may encounter.

Bug: 14975160
Change-Id: I39848d5666f9083cb4eca493e5cdaa868f3f99fb