[SB][Notif] Don't show `when` time in chip if it's now-ish or in the past.

Most apps set their `when` to just be the current time (or they don't
set it at all and system_server automatically fills in the time as the
current time), which would cause the status bar notification chip to show
"now". "now" probably isn't correct for most of these apps, so just show
nothing if the `when` is now-ish or in the past.

Fixes: 393369213
Bug: 364653005
Flag: com.android.systemui.status_bar_notification_chips

Test: Send promoted notification with `when` in future -> verify chip
shows time
Test: Send promoted notification when `when` equal to current time ->
verify chip doesn't show any text
Test: Send promoted notification with `when` in the past -> verify chip
doesn't show any text

Test: atest NotifChipsViewModelTest

Change-Id: Ia585542a8fd67a49f09b60eced0f6d5c9a092021