vp9: Allow for disabling loopfilter per spatial layer

vp9: Allow for disabling loopfilter per spatial layer

For SVC: add parameter to the control SET_SVC_PARAMS to
allow for disabling the loopfilter per spatial layer.
Note this svc setting will override the setting via
VP9E_SET_DISABLE_LOOPFILTER (which should only be used
for non-SVC).

Add unittest to handle both SVC (spatial or temporal layers)
and non-SVC (single layer) case.

Change-Id: I4092f01668bae42aac724a6df5b6f6a604337448