[E2E test] Add an assertion that we exited DW mode after closing the last app

Bug: 364590923
Flag: EXEMPT no flag is needed for instrumented test
Test: atest PlatformScenarioTests:com.android.wm.shell.functional.CloseAllAppsWithAppHeaderExitTest

Change-Id: I4bd05e07ed414186a4c9fb0687ad4d28cd0297c8
1 file changed