Make d8-on-eng a per-module opt-out

Several modules use defaults that set `enabled`, rather than relying on
the defaults.  With this change, eng builds no use d8 instead of r8,
with the exception of unit tests in prebuilts/r8.

Bug: b/374975543
Test: manual, TH
Change-Id: I377d354f005529d04fb9d9255afa73979935f091