vp9: Rate control adjustments for screen content.

vp9: Rate control adjustments for screen content.

For screen content mode: changes to reduce occurence of
significant QP decrease (from one frame to next),
which can cause large frames (overshoot/delay).
-cap the buffer increase to optimal level for frame drop
mode where full superframe can drop
-reduce the max_adjustment_down due to buffer overflow
-reduce qp threshold to trigger re-encode on large frame

Change-Id: I3e30e4814192b5f728abff3f7359eb64f561b8f0