Add /file_contexts.bin only to devices which need it

/file_contexts.bin is needed only by devices which do not have
PRODUCT_FULL_TREBLE set to true. Adding this file to devices which
have PRODUCT_FULL_TREBLE set to true causes confusion and wastes
about 800 kB of space.

/file_contexts.bin was being added unconditionally to all devices due
to the build system needing the combined file_contexts file for
host-side labelling of filesystems in images and for inclusion into
target files ZIP (again, to be used for host-side labelling later).

This change switches these targets to depend on the same file, but
from intermediates directory. As a result, everything continues to
work just fine, but without pulling in /file_contexts.bin. On devices
which need /file_contexts.bin, it is pulled in by
system/core/init/Android.mk.

Test: /file_contexts.bin still there on bullhead, but not there on
      sailfish with PRODUCT_FULL_TREBLE set to true.
Test: Clean build, device boots up with, no new denials.
      Reboot to recovery, recovery boots up fine too, no denials.
      This was tested on sailfish-eng and marlin-eng.
Bug: 36002414

Change-Id: I9bbbb08bbf7d874bba0bafcc19bcbf9eec564326