vp9: Add flatness metric to cyclic refresh setup.

vp9: Add flatness metric to cyclic refresh setup.

For screen-content with aq-mode = 3: identify spatial
flat superblocks in the setup stage and don't mark them as
candidates for refresh. Spatially flat blocks are already
removed from refresh at a later stage in the encoding (in pick_mode),
but doing this at the setup stage of cyclic refresh (before encoding)
allows refresh to more quickly hit the text areas. Only drawback is
an extra source variance calculation for a set of superblocks on
each frame.

Adjust the refresh rate: lower it to reduce overshoot since
more texture areas are hit faster with this change.

Change-Id: I88fa20e52fdbf1a938ae814f9b48c887f1f909d2