Introduce MotionAccumulator in MagnificationGestureDetector

This is a pure refactoring of MagnificationGestureDetector so that
motion event's coordinate calculation is encapsulated in a new interface
MotionAccumulator.
This makes it easy to customize the behavior for input events in the
following chage.

Bug: 420776720
Test: MagnificationGestureDetectorTest
Flag: EXEMPT refactor
Change-Id: Id47f7ae3dfcc6f2d1eb3c98bbc35cecfe855f009