Add aconfig flag for new magnification cursor following

Fullscreen magnification's cursor following will migrate to use a new
input framework.
This flag will be used for the migration and implementation of 3 types
of cursor following behaviors.

Bug: 361817142
Test: build
Flag: com.android.server.accessibility.enable_magnification_follows_mouse_with_pointer_motion_filter
Change-Id: I7c1aa17db9026d71afd26db594a3dc8ea0abfd89