Slightly change the way we save and reload frame contexts during
frame setup. For "normal" frames everything is the same, but for
error-resilient and/or intra-only frames, we now:

* Reset the frame context using setup_past_independence()
  (+ extra code if q-adapt-probs is enabled), as usual
* Store this frame context into a special slot in cm->frame_contexts
* Use that slot to fill in cm->pre_fc

The main difference from before is that (for error-resilient/intra-only
frames which are not key frames) we used to throw away the frame
context after setting it up, and would re-use whatever was set up
at the last keyframe.
This was fine when q_adapt_probs is disabled, but it caused an
inconsistency when combined with q_adapt_probs. See the attached
bug report for more details on that.

BUG=aomedia:1104

Change-Id: I9532b6b0e8ae29efbb4f059a0c67a73d7c7828ce