Fix ObserveFirstPassMotionVectors()

Fix ObserveFirstPassMotionVectors()

1) Use kRefFrameTypeNone in the unit test
2) Reset mv_info in fp_motion_vector_info_init
3) Call fp_motion_vector_info_init() in first_pass_encode()
4) Set mv_info for intra frame.
5) Set mv_info with zero mv as default for inter frame
6) Remove duplicated fp_motion_vector_info in encode_frame_info

Change-Id: I2f7db5cd4cf1f19db039c9ce638d17b832f45b6e