Set refresh_alt_ref_frame on keyframe encode.
On a keyframe alt ref and golden are refreshed. The flag was not being set and so on the frame after a keyframe, motion search would occur on the alt ref frame. This is not necessary because the alt ref frame identical to the last frame in this scenario. Handle corner case where a forward alt-ref frame is put directly after a keyframe. Change-Id: I9be4cf290d694f8cf2f9a31852014b5ccf1504d3