Implement SmartReplyView

SmartReplyView creates and manages smart reply buttons in a
notification.

One-pager:  http://go/1p-smart-reply
PRD:        http://go/srn-prd
Mocks:      http://go/p-notifications
Design doc: http://go/srn-design

Screenshot: https://screenshot.googleplex.com/GT9NGckkFKB.png

Bug: 67765414
Test: Viewed a notification with mock smart replies
Change-Id: I6d90776571a559a2542aa35fa11e6a21969179c0