Create one _system and one _nonsystem target, which together contains
the same artifacts as before, just split by whether they go on the
system partition or not.

The product build hierarchy is being refactored to be split by
partition, so these targets facilitate inclusion of just the
system parts where necessary. Also keep the selinux_policy target
around for products that don't need the split.

Bug: 80410283
Test: for t in eng userdebug user; do lunch mainline_arm64-${t}; m nothing; done
Test: verified walleye /system and /vendor identical before and after, via:
Test: /google/data/rw/users/cc/ccross/bin/compare-target-files.sh P6259983 walleye-userdebug "SYSTEM/*" "VENDOR/*"
Test: only diffs are in build.prop files (timestamps and the like)

Change-Id: I0f5d8a1558a164ce5cfb7d521f34b431855ac260