Make system partition import system_ext's aconfig flags

This is the behavior in make since aosp/3261300. It's a problem for
treble though, so maybe should be revised in the future.

If the system -> system_ext dependency becomes a problem (like if we
need to add a system_ext -> system dependency later), I think we could
break it up by creating a ".aconfig" sub-module of filesystem modules,
and then the dependency is only added from system -> system_ext.aconfig.
But we don't need that right now.

Fixes: 382518797
Test: diff out/target/product/vsoc_x86_64/system/etc/aconfig_flags.pb out/soong/.intermediates/build/soong/fsgen/aosp_cf_x86_64_phone_generated_system_image/android_common/system/system/etc/aconfig_flags.pb
Change-Id: Ia0d043e35f03bbf2bc8a29df0b2b8ecd8427e727