Make mainline_system_arm64 build only /system

Configure this product not to build any of the other partitions.

Note that the only partition that currently has artifacts produced
is system_other, and this change doesn't appear to prevent odex/vdex
files from being created there anyway. However, it does allow
configuring the target device (mainline_arm64) to have more partitions
in a followup change.

Bug: 111533333
Test: make
Change-Id: Ib115cb03bc99b9b61e0d6f1b622784e7ad5fcea0