Custom SmartReplyView layout

Implement SmartReplyView#onMeasure and SmartReplyView#onLayout according
to the redlines in go/p-notifications:

  * Smart reply buttons can have at most 2 lines of text.
  * Squeeze buttons to 2 lines of text when necessary to fit more
    buttons.
  * Don't show buttons which have more than 2 lines of text or don't fit
    within the notification width.
  * Update button background and text color.

Screenshot: https://screenshot.googleplex.com/cSM6Ve7qjb3.png

Bug: 67765414
Test: atest SmartReplyViewTest
Change-Id: Ia161c7f5669e7aef1b8c3480a8836784f0bde055