Add end-to-end test for full screen magnification continuous mouse following

This adds an integration test for full screen magnification's mouse
following feature of continuous mode.
The test injects virtual mouse input event and verifies observed
magnification spec from accessibility service.

Bug: 361817142
Test: FullScreenMagnificationMouseFollowingTest
Flag: com.android.server.accessibility.enable_magnification_follows_mouse_with_pointer_motion_filter
Change-Id: I8c03c3012abd2aa39fc85c9992ce5c1ceb12fb73