put spatial svc behind an ABI check

this removes the CONFIG_* checks from public headers, but means
'--enable-experimental --enable-spatial-svc' builds will fail without a
local change to the ABI in vpx_encoder.h. this should be all right for
testing this experiment.

Change-Id: Ief55e7b9d1e8332cfce990275e04c29b30af0c4a