Reapply "Use partition intead of "root" to assemble filesystem artifacts"

This reverts commit 13590446.

The revert will be applied alongside a change in build/make that ensures
that the correct intermediates dir for aosp_shared_system_image is
packged into target_files.zip

Test: Previously failing OTA test now passes https://android-build.corp.google.com/builds/abtd/run/L04100030008317770
Change-Id: Ibdf3ae42348ec5840dcada8ce3182f87d4619cac