Make update_map/temporal_update fields implicit for keyframes.

These frame types cannot make bitstream parsing depend on previous
frames, so the hypothetical combinations of e.g. keyframe=1 and
update_map=0 or keyframe=1 and temporal_update=1 are non-sensical.
Therefore, make it impossible to code such combinations in the vp10
bitstream header.

See issue 1044.

Change-Id: I3f0a83d5c7e3989541a469a909471424a285239d