Fix sensitive content Toast for multiple windows in same activity

The current logic for onWindowVisible only triggers for the first window
of an Activity. Moving to relayoutWindow makes it trigger reliably.

This behavior is already tested through
ViewSensitiveContentTest#testToastIsShown since that shows a splash
screen when launching the protected Activity. The test is currently
flaky.

Bug: 323580163
Bug: 332206566
Test: manual
Test: atest CtsSensitiveContentProtectionTestCases --iterations
Change-Id: I6170125410d27c8be48c403fc91e0025762ade48