Disable Immersive Confirmation Dialog in FullScreenMagnificationMouseFollowingTest

In this test, an activity is made immersive fullscreen so that the
entire screen is accounted for magnification.
Making an activity immersive may trigger showing an immersive
confirmation dialog.
This change disables showing the dialog to stabilize the test.

Bug: 361817142
Test: FullScreenMagnificationMouseFollowingTest
Flag: com.android.server.accessibility.enable_magnification_follows_mouse_with_pointer_motion_filter
Change-Id: I4caebaebf38d90316a1980669be2e5e0705bc1e5