Forcefully clean up apps on teardown()

This CL changes teardown() method to clean up used apps forcefully
rather than closing apps gracefully via close button.
The purpose of the tearing down process is to clean up everything after
the test run, so even when the close button is not working (e.g., SysUI
crashed) the apps should go away forcefully.

Flag: EXEMPT - test fix
Bug: 440453135
Test: ConnectedDisplayCujSmokeTests

Change-Id: I4c4e27b92c11c4c2b32a4e9c0fd196c2b910d9d8