APEX extraction rule mentions APEXes

so that it's triggered even when APEX list shrinks.
(-d explain will print "command line changed for ...")

To repro:
- Pick one VAPEX with vintf fragment.
- Copy the fragment to /vendor/etc/vintf/manifest (DEVICE_MANIFEST_FILE)
- `m check-vintf-all` should fail
- Remove the APEX from PRODUCT_PACKAGES
- `m check-vintf-all` should pass

Bug: 304769672
Test: m check-vintf-all
   (see above)
Change-Id: Iae9da91105faebfb8a5127fbb02a414b9ffdca10