Fix NPE when run the splitscreen test

ccfed74e, add the toast maker in StageCoordinator. NPE is
thrown when running the splitscreen test. It can't toast on a
thread that has not been called Looper.prepare(). This CL adds the
@UiThreadTest on needed tests to fix the NPE.

Bug: 256087248
Test: atest com.android.wm.shell.splitscreen
Change-Id: I9fef85c58570a9c1ca2741d1a8aa81439850cc88