SystemUI: screenshot: remove duplicate clamping on move action

On MotionEvent.ACTION_DOWN, a range of of motion is calculated and
stored. On MotionEvent.ACTION_MOVE, the new position of the crop
boundary is clamped by the original range of motion, and also
clamped again by a new range of motion inside setBoundaryPosition().

The original range of motion is pointless and only used in this case,
remove it.

Change-Id: Ib4f35bf5317f5b5879606680beee3e077f829d10