libvpx

Improve golden frame refreshing in non-rd mode

The default golden frame interval was doubled. After encoding a
frame, the background motion was measured. If the motion was high,
the current frame was set as the golden frame. Currently, the
changes were applied only while aq-mode 3 was on.

Borg tests(rtc set) showed a 0.226% PSNR gain and 0.312% SSIM gain.
No speed changes.

Change-Id: Id1e2793cc5be37e8a9bacec1380af6f36182f9b1