[Snowflake] Use button widget and button bar widget for modal dialog

+ Change buttons to ButtonCompat
+ Change button bar to DualControlLayout so that it is more consistent
  with other button bars
+ Adjust padding to match the current spec

Bug: 856244
Change-Id: I8bd1d211a27fa47769286cf565a627da65657f57
Reviewed-on: https://chromium-review.googlesource.com/c/1351790
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611222}