[Expressive design] Update SPA Appbar.

Update container color.
Update back icon size.
Screenshot: https://screenshot.googleplex.com/9KqVsoj2uv7Pv5a

Bug: 349681531
Flag: EXEMPT migration
Test: visual
Change-Id: I94e98d00819926ba584d15d4df6540142e602361
3 files changed