a11y: Throttle onMouseMove in MagnificationController

This change introduces a 300ms throttle to the onMouseMove event
handler in MagnificationController. This prevents the handler from
being called too frequently during rapid mouse movements, which can
cause performance degradation.

New unit tests are added to verify that events are correctly
rate-limited and that they are processed again after the throttle
period has passed.

Bug: 424272057
Flag: EXEMPT bugfix
Test: atest MagnificationControllerTest
Test: atest IMagnificationConnectionTest
Change-Id: I79d959631963612d861333f708dccf193529beac