app/greeter:move the pointer to the screen center

problem:
    in Mult-Monitors,the pointer is in the full screen center,
    but because the pointer is LEFT_POINTER,
    so it same to be in the next monitor.

solve:
    move the pointer to the center of the monitor which the container in

tower:https://tower.im/projects/1c0cd0c59ef941298c6e6b2ba6833b91/todos/e8c8fe9ae15042bba0c8db060843fab3/

Change-Id: If13e906c64d10d1cbb516bf08d84f21d764cf35a