Ignore 'when's of 0

And treat them as current time, instead.

Also move all code to use the single feature flag because it's hard to
keep both flags in sync.

Bug: 330193582
Flag: android.app.sort_section_by_time STAGING
Test: NotificationTest
Test: view system USB notifications on device
Change-Id: I01efc63ddd1a60b69ad284c7e8120fa6ddc6dcf0