Use notification "when", not sbn post time, for redaction

Apps will sometimes repost old messages as a part of new notifications,
causing re-redaction for already-seen OTPs.

Bug: 387452286
Test: NotificationLockscreenUserManagerTest
Flag: android.app.redact_sensitive_content_notifications_on_lockscreen
Change-Id: I41454df6abcb73d0845a769d76245d054ed2d787