Add missing setup-test-rule to rest of Desktop e2e scenarios

All Desktop e2e scenarios should use the rule Utils.testSetupRule() to
ensure each tests starts with a clean slate (i.e. we're not in Desktop
mode).
Also fail in DesktopModeAppHelper#enterDesktopMode() if we're already in
Desktop Mode as that's an unexpected state.

Bug: 421120649
Flag: EXEMPT e2e test changes
Test: atest
Change-Id: Ic7f3b8b2a37bf8f34864ee4bafde05f6b2fd15de