Variables uses vintf_fragment_modules

Product variable cannot be resolved when vintf_fragments are translated
into vintf_fragment modules, because product variables are being handled
after the translation phase. This change updates variables to use
vintf_fragment_modules instead of vintf_fragments so it can be handled
without resolving depenency between two mutators.

Bug: 322089980
Test: aosp cf build succeeded
Change-Id: I224ddb05bd86e24ae220af616b5a89accd043dd2