Set theme preview text shadow color to transparent when text is dark

There's still black shadow for dark text: https://screenshot.googleplex.com/2CA5jmCf9ms.png
Launcher sets transparent shadow for the dark text: https://screenshot.googleplex.com/Y4L0isk8dp4.png

After:
Dark text: https://screenshot.googleplex.com/mqQ6DhJpQtj.png
Light text: https://screenshot.googleplex.com/hasi7K964Ew.png

Test: Manually
Fixes: 157617856
Change-Id: I9b50b124c7eb1e7bbf488ee3c299a51417727f90