Add Notification.Builder.setAllowSystemGeneratedContextualActions().

If an app doesn't add any smart replies or smart (contextual) actions to
their (message) notifications the Android platform fills in smart
replies and actions for the app.
With this CL we add a way to opt out of system generated contextual
actions.

Bug: 119765729
Test: atest NotificationContentViewTest
Change-Id: I6190fd197f74b8353f7113f2d8ba86068c2b78ab