Makes Flash Notifications preview broadcasts be protected

Flash Notifications preview broadcasts should not be sent by non-system apps for now. Makes them be protected to ensure no malicious apps can impersonate the OS to perform the unexpectly flashing. Also, broadcasts sent from system internals should be protected to avoid security holes.

Bug: 266775677
Test: ensures non-protected broadcast from system error logs are gone manually
Change-Id: Ibd8b76d505e65f8150619607545e87d550e6affb