Measure absolute value of pointer motion distance

If pointer is moving to the negative axis direction, pointer movement
distance will always less than pre-defined mMotionSlop, thus
TAP_OUTSIDE_STACK will have chance to be sent, will cause unnecessary
focus stack switch.

Change-Id: Ia4c066bebce250257c7361ef976907a0ca2c6461
Signed-off-by: tingna_sung <tingna_sung@htc.com>