Simplify MouseEventHandler construction

This simplifies MouseEventHandler construction so that it's
created inside FullScreenMagnificationGestureHandler, and there's no
need for A11yInputFilter to know about it.

Bug: 361817142
Test: FullScreenMagnificationGestureHandlerTest AccessibilityInputFilterTest
Flag: EXEMPT refactor
Change-Id: Iba9ec12c8e9afacaf4522533292967c8e59b549d