Only output make targets for uninstallable static libs in the APEX

unavailable-to-platform case.

This removes the special case added in https://r.android.com/1274763
from SkipInstall(), so that it doesn't cause conflicting AndroidMk
entries when a cc_prebuilt_library_static module has prefer:true and
the corresponding source module exists.

Test: `m` in a tree with a snapshot created from art-module-sdk where
  the libartimagevalues module has prefer:true
Bug: 151303681
Change-Id: I651ae325753b707296892adb4cae80daaddb6af2