a11y: Fix FullScreenMagnificationMouseFollowingTest

Fixes flakiness caused by unexpected device setting states. It also adds
more logging for falure diagnosis.

Bug: 422242864
Flag: com.android.server.accessibility.enable_magnification_follows_mouse_with_pointer_motion_filter
Test: FrameworksServicesTests:com.android.server.accessibility.integration.FullScreenMagnificationMouseFollowingTest
Change-Id: Ic0f8d3d579e2a27a5a4ae61587b15f4dcd32fba5