Moves the redfin FCM back down to level 5.

redfin's manifest.target-level will stay at 5 even as the vendor is
updated, so this FCM shouldn't have been moved to level=6.

This change includes conditional logic to exclude the S-based level=5
FCM from the build when creating a Vendor-Freeze-compatible system-only
build.
This allows VF system builds to specify their own level=5 redfin FCM
(compatible with the older frozen R vendor) that differs from this
level=5 FCM (which is compatible with S vendor).

Bug: 184976701
Test: build redfin-userdebug and redfin_vf_r-userdebug in Forrest.
      Observe check_vintf logs.
Change-Id: Ie25de43fec6fbf0714740a5b7803dd60fd164d41