Test sensitive content Toast doesn't reshow for Notification case

The BeforeAppLaunch and AfterAppLaunch cases are sufficient to exercise
the behavior fully.

Also moves the existing Toast verification into the ActivityScenario
launch block, so that it isn't delayed by the Activity being destroyed.

Bug: 323580163
Test: atest CtsSensitiveContentProtectionTestCases --iterations
Change-Id: Ibf5fbb24e8c492d710e122d59ce5aff4ab95769a