Updates to Adjustment.KEY_IMPORTANCE handling

- Do not run signal extractors on enqueued notifications - the
assistant isn't told that information and the information that
the extractors base their decisions on it's final. Instead,
extract signals in the post runnable, before the sort/before we
generate the notification update for listeners

- Ignore importance adjustments sent from onNotificationEnqueued
that arrive after the notification was posted, because it's too
late to cancel the notification or silence it.

- Also fix a bug in the handling of importance adjustments
that are sent via adjustNotification: *listen* to the comment that says
'any field that can be changed in an extractor needs to be handled
in handleRankingSort'

Change-Id: I1603d6c5701fcc16f038231e22e21565dd366572
Fixes: 120601946
Test: atest, manual