Don't retain the filtered list of notifications.

Previously the entire set of notifications was "leaked" until
the next call to getNotificationsForCurrentUser(), which
currently only happens for certain configuration changes
(UiMode, density, fontscale).

Bug: 143461605
Test: atest SystemUITests
Test: sanity check: boot and switch night mode
Change-Id: I39f2c96f848ab92a050aa4bda3ae203fbf99d915