init: 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 creates two phony
packages to achieve this correctly in the build system.

Bug: 117606184
Test: mainline builds work properly
Change-Id: I19263e76c217c2e70bc06c180be3a460e9144f18