Refactoring for magnification [1/n]

This change doesn't change any user behavior.
It's refactoring that reduce the common code and logic.
1. Adds the common varibles to the parent class, MagnificationGestureHandler
for fullscreen and window MagnificationGestureHandler.
2. Moves the implementation of  onMagnificationScaleChanged() from
AccessibilityManagerService to MagnificationController.

Bug: 174291719
Test: atest AccessibilityManagerServiceTest FullScreenMagnificationGestureHandlerTest
  MagnificationControllerTest WindowMagnificationGestureHandlerTest
Change-Id: Ic4d6c966600a113fd06e7165353e014aca3189bf