Soong filesystem generates aconfig files for subpartitions

When system had system_ext or other subpartitions, aconfig files for
the subpartitions were missing. Generate those aconfig files for the
subpartitions.

With this fix, we don't need to import aconfig files from system_ext
to generate system aconfig files.

Bug: 385213580
Test: lunch gsi_x86_64-trunk_staging-userdebug && m
Change-Id: Ie3db5c73c50af97f3ad0bea5e0c84273c9fce886