Tweak the sizes of the smart reply buttons

Tweak the sizes of the smart reply buttons to be more consistent with
the rest of the notification UI.

Note that the padding for two-line buttons needs to be different from
the padding for one-line buttons until b/78876518 is fixed.

Screenshot before: https://screenshot.googleplex.com/ePaB3YZfLEk.png
Screenshot after: https://screenshot.googleplex.com/vZmPq7AyYOB.png
Two line version: https://screenshot.googleplex.com/9o8PxMusDXV.png

Bug: 73605520
Test: In Notify post a Messaging notification with chices
Change-Id: I4ef16dc8fb2682f9ac1a74aecadf8b6b6799cfb3