apex: file_contexts for override_apex flattened

Make-var APEX_FILE_CONTEXTS_INFOS should contain entries for
override_apex modules. Otherwise flattened override_apex modules can't
be correctly labeled.

Bug: 154031888
Test: lunch gsi_gms_arm64-userdebug
      m file_contexts.bin
      check ../file_contexts.bin_intermediates/file_contexts.concat.tmp
      if it contains entries for override_apex
Change-Id: I60fc3da1def0f5fe14cfeef32a4c2a07a65b056e