Make MoveToNextDisplay fail with assumption failure

Since MoveToNextDisplay is an end-to-end test, it cannot override flag
values. Instead, it should fail with an assumption failure if the
required flag is not enabled.

Bug: 385032739
Flag: com.android.window.flags.enable_move_to_next_display_shortcut
Test: atest PlatformScenarioTests:com.android.wm.shell.functional.MoveToNextDisplayTest
Change-Id: I3448829443d7698f5b3a1d715ed5c1e09efdffef