Add smart actions to message notifications.

Smart Action look similar to Smart Replies with an icon next to the
button text.
Smart Actions can be added either by an app, or by the framework itself
(through NotificationAssistantService).
The idea is that Smart Actions are really actions relating to the
content in the notification message.

The priority order of smart replies vs. actions will be dealt with in a
follow-up CL (i.e. whether we should prioritize showing smart actions
over smart replies).

Example notifications:
System generated:
https://screenshot.googleplex.com/f6p8uOMygeT.png
App generated:
https://screenshot.googleplex.com/VR6On1eh4CW.png
2 app generated actions:
https://screenshot.googleplex.com/QuM5e792xhb.png

Bug: 111406942
Test: manual tests = add custom actions directly in sys ui and ensure
they're shown in message notifications.
Test: atest SystemUITests

Change-Id: I0ce959b74e89bcabc69b76c50c1877e70d6f06df