Update FullScreenMagnificationMouseFollowingTest

This refactors FullScreenMagnificationMouseFollowingTest to use
DesktopMouseTestRule so that we don't need to have our own logic to set
up virtual devices.

Because the rule currently supports mouse only in default display only,
the test is updated to use default display.

Different from a virtual display, default display has insets from cutout
and system bars. In the test, the activity hides all of them so that
magnification region matches the display size.

This also relaxes the floating error tolerance because Linux evdev
support only int delta, thus at most 1 px error is allowed in the test.

Bug: 401146470
Test: FullScreenMagnificationMouseFollowingTest
Flag: com.android.server.accessibility.enable_magnification_follows_mouse_with_pointer_motion_filter
Change-Id: Iafb27f80da397a84d7811b0741787542f5b0f189