SystemUI: fix notification redaction logic.

Previously, the logic for redacting sensitive notifications is flawed
for apps that have individually overridden the system-wide redaction
setting. Sensitive notifications from such apps are still redacted even
after unlock.

This CL fixes the issue and also simplifies the redaction logic by
taking advantage of the needsRedaction() method.

Bug: 35951989
Test: manual, see bug log
Change-Id: I6b72ed97b3056a93cbe44f54c607c16445f39551
(cherry picked from commit 0d9f7d7b)