Default 'showWhen' to true

Although Notification was changed to default it to false in Android N,
NotificationCompat was not updated to match. This is guarded by the flag
that controls sorting changes, since showing times matters more when
thee shade is sorting by time.

setWhen is also now ignoring inputs of "0" when 'showWhen' is true, to
avoid showing timestamps that are decades old. This value is already
ignored by the sorting logic.

Test: manual
Bug: 330193582
Flag: android.app.update_ranking_time DEV
Change-Id: Ibcf9dacc618871d37ddbdd3c16d23fde12650bce