Post callback onSecureWindowShown on the main thread

onSecureWindowShown will trigger a toast showing and might cause
deadlock since showing a toast will acquire ActivityManagerService
lock. Post onSecureWindowShown on the main thread, so it doesn't
block activity launching.

Bug: 244139910
Test: Manually test with Exo on multi-display environment
Change-Id: Idf29b534a3657a265a5f389e0b388bc3af19c04e