System-specific product sysprops

This change makes it possible for products to specify the values
of the ro.product.system.X sysprops independently from the
corresponding sysprops on the other partitions.

Leave the fingerprint as-is for now. It will be changed to follow
suit in a followup change.

Bug: 110206836
Test: make
Change-Id: Id30012e1948df792778b102203116d4ae3f68e56