multi-res: restore v1.0.0 API

Move the notion of 0 bitrate implying skip deeper into the codec,
rather than doing it at the multi-encoder API level. This preserves
v1.0.0 ABI compatibility, rather than forcing a bump to v2.0.0 over a
minor change. Also, this allows the case where the application can
selectively enable and disable the larger resolution(s) without having
to reinitialize the codec instace (for instance, if no target is
receiving the full resolution stream).

It's not clear how deep to push this check. It may be valuable to
allow the framerate adaptation code to run, for example. Currently put
the check as early as possible for simplicity, should reevaluate this
as this feature gains real use.

Change-Id: I371709b8c6b52185a1c71a166a131ecc244582f0