Add biasing to ZEROMV for videos with static background

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