vp9_ratectrl: change ARF overlays rate correction factor

ARF overlays now use the same rate correction factor as regular inter
frames, further testing would be needed to see if it makes sense to use
a completely separate rate correction factor for ARF overlays.

$ vpxenc --cpu-used=5  --fps=50/1 --target-bitrate=2000
         parkjoy.y4m -o out.webm
=> Before: 3356 kb/s
=> After: 2271 kb/s

Change-Id: I73e4defa615ba7a8a2bdb845864f4b1721cbbffe