Add soong module only for aosp_{arch}

aosp_{arch} build GSIs for release, whereas gsi_{arch} build GSIs for
compilance test. System images of aosp_{arch} are supersets of system
images of gsi_{arch}, with a few exceptions (userdebug sepolicy).

This adds a Soong module for aosp_{arch} targets, named
aosp_system_image, enabled only when building GSI.

This change is a reland of ag/31654647 where the build is broken on
targets of downstream branches which inherit aosp_{arch} targets.

Bug: 381019944
Test: build, TH
Change-Id: I390d59cb4168eee8c422a4c1b257ade465d17921