Refactoring for magnification [2/n]

This change doesn't change any user behavior.
It's refactoring that reduce the common code and logic.
1. Moves the common logic of onMotionEvent to the parent
class, MagnificationGestureHandler.
2. Moves some of the debug flags to the parent class.

Bug: 174291719
Test: atest com.android.server.accessibility.magnification
Change-Id: If0a4a5e934a650b40e064dc3a0b72ce68780409b