Incorrect file names in apkcerts.txt for Make modules

This change fixes a regression that was caused by I56bc55e2231eb946617a9d6f97d55c9b61f3bff7
that file names for Make modules are ".apk" in apkcerts.txt.

This was because PACKAGES.$(LOCAL_MODULE).STEM was only set for the
modules from Soong. It is now set for Make modules too.

Bug: 155440232
Test: OVERRIDE_TARGET_FLATTEN_APEX=true m apkcerts-list
and check the generated apkcerts txt file to see that it doesn't have an
entry where file name is ".apk"

Change-Id: I6f14bbb5678255bef2c1f2397f59ede6c2b5df18