libvpx

Limit cyclic refresh on steady background blocks.

Use the existing QP condition on limiting cyclic refresh, and add
addiitonal condition that block has been encoded with zero/small motion
x frames in row (where x is at least several times the refresh period).
Additional condition only affect non-screen content mode.

This helps to improve visual stability for noisy input, where on steady
background areas the application of delta_qp may lead to encoding the noise.

Also added a change to use the true skip (after encoding) to update the
last QP.

Change-Id: I234a1128d017d284cf767fdb58ef6c59d809f679