Fix MoveToNextDisplay e2e test failure

In MoveToNextDisplay#tearDown(), the test relaunched the test activity
on the default display. This was a workaround for b/405234435.

However, before this fix, relaunching the Activity with
`testApp.launchViaIntent` would fail. It appears
`testApp.launchViaIntent` is no longer able to relaunch an Activity on
the default display if the Activity already exists on another display.

This CL explicitly exits the Activity on the simulated display, allowing
the subsequent relaunch on the default display to succeed.

Bug: 418619628
Test: atest WMShellFlickerTestsDesktopMode:com.android.wm.shell.functional.MoveToNextDisplayTest
Flag: com.android.window.flags.enable_move_to_next_display_shortcut
Change-Id: I40999dd3aadeb3c10c8cd00f2adda7886479ed65