Add shadow for theme preview workspace texts

Also make the app name text smaller to match the icon scale.

Screenshot:
Launcher https://screenshot.googleplex.com/EDCmpVYQ7b8.png
Full preview https://screenshot.googleplex.com/7JtXHwoqSrQ.png
Compared with bug:
Before https://screenshot.googleplex.com/0ifPX9NycWH.png
After https://screenshot.googleplex.com/3cb2fKBoSYC.png

Test: Manually
Fixes: 157617856
Change-Id: Ia4d21adfc16a28a49be762f15aae23aa83606613