Avoid re-showing Toast for sensitive content screen capture blocking

While an app stays in the block list, avoid showing the Toast every time
its blocked window becomes visible.

Bug: 323580163
Test: manual
Test: atest CtsSensitiveContentProtectionTestCases:android.sensitivecontentprotection.cts.ViewSensitiveContentTest --iterations
Change-Id: If89ff070ae1fe4b47bd588410a36ed4b8e372931