Add Smart Suggestions to heads up notifications.

Add smart replies and actions to Heads-up notifications (HUN).

Note: only one line of text is shown in HUNs with messaging templates.
Thus the user might not see the full message to respond to even when the
smart replies/actions are displayed.

Screenshot: https://screenshot.googleplex.com/3R5GVZGXGNg.png

Bug: 117257685
Test: Use cinek@'s Notify app to display HUN with smart replies/actions.
Change-Id: I10d2c87b445de8f471ad0978829cede9ac6a6663