Simplify SmartReplyView padding layout logic

The horizontal padding used to be dynamic based on the number of lines, but that hasn't been true for a while.  The new goal is for padding to always be 16dp, except that the action icon is in the middle of that.

Fixes: 78876518
Test: manual visual inspection with notify
Test: atest SmartReplyViewTest
Change-Id: I11400a552cab99d9f0ae31024d5973772e8ea300