Don't create nonplat_service_contexts on full_treble devices

On full Treble devices, servicemanager should only host services
served from processes on /system; nonplat_service_contexts
should not be created at all in this case.

Bug: 36866029
Test: Build marlin and walleye and make sure nonplat_service_contexts
      is not created.

Change-Id: Id02c314abbb98fc69884198779488c52231d22c3