a11y: Link cursor following mode backend with user preference

This makes sure the cursor following mode value is populated to the
pointer motion event filter. It links the user preference with the
backend.

This also moves the pointer motion event filter from
`FullScreenMagnificationController` to `AccessibilityInputFilter`. The
reason is that we do not need one event filter per display, and it's
easier to manage at `AccessibilityInputFilter` level.

Bug: b/355503630
Bug: b/355727440
Flag: com.android.server.accessibility.enable_magnification_follows_mouse_with_pointer_motion_filter
Test: FrameworksServicesTests:com.android.server.accessibility.AccessibilityInputFilterTest
Test: FrameworksServicesTests:com.android.server.accessibility.AccessibilityManagerServiceTest
Test: FrameworksServicesTests:com.android.server.accessibility.AccessibilityUserStateTest
Test: FrameworksServicesTests:com.android.server.accessibility.magnification.FullScreenMagnificationControllerTest
Change-Id: I044e735b75db3452a3c3e016c1325783ddb8a667