Disallow accessing Treble _OVERRIDE reqs.

If something needs to distinguish between _OVERRIDE and non-_OVERRIDE,
it is a hack (these should be the same property).

Bug: 114488870
Test: m nothing (exercises this)
Test: searching all of these reqs in the code.
Change-Id: Idc22150a3179d025d7d95beafad39e8b07946ef8