a11y: Show magnification mode switch button on mouse move

This change enables the magnification mode switch button to appear
when the user moves the mouse, creating behavior consistent with
existing touch interactions.

It adds the necessary logic to MagnificationGestureHandler and
MagnificationController to detect mouse hover events and trigger the UI.

Additionally, this change includes unit tests for the new functionality
and improves existing related tests:
  - Added tests to verify onMouseMove is called correctly.
  - Added negative test cases to prevent regressions.

Bug: 424272057
Flag: EXEMPT bugfix
Test: atest MagnificationGestureHandlerTest
Test: atest MagnificationControllerTest
Change-Id: I53dfd21dac4f8684a473aa9e60d3b58192065e3e