Add a functional test for opening app with display connected

This CL adds a functional test to open an app on the default display
when an external display is connected. It should result in opening the
app in desktop mode.

Bug: 391473511
Flag: EXEMPT add test
Test: atest PlatformScenarioTests:com.android.wm.shell.functional.OpenAppWithExternalDisplayConnectedTest
Change-Id: I0e9ab91d75620e781e783922cb9dd1f2f4f6997c