Separate init packaging for mainline

With system-as-root, both first and second stage init belong to
system, but without system-as-root, first stage belongs to vendor and
second stage belongs to system.  This change references two phony
packages that include the right parts of init for systen and vendor.

Bug: 117606184
Test: mainline builds work properly
Change-Id: I997309655fef28cb1c3d4b3be989c035b5c6488d