Reject old notifications

Reject notifications with a 'when' that's 2 weeks or older.

An unset when and a when of '0' are special cased and will still be
posted as those appear to users as current notifications

Test: NotificationManagerServiceTest
Flag: com.android.server.notification.reject_old_notifications
Bug: 339833083
Change-Id: I62254e7e86194eda1e6f1a8b5e35181bcb4ecf31