Add biasing to ZEROMV for videos with static background

For videos with big static background(such as video conferencing
clips), the mode decision was biased to ZEROMV in order to
obtain a stable background. The percentage of ZEROMV on last
frame was used to predict if there is static area in current frame,
and checking already-encoded neighboring macroblocks' motion
vectors to make sure the local area has low motion.

Change-Id: I05b3241d3a56a0bda88b6681e5646c1c8baf2e57