gd: Fix focus ring in Game Dashboard Button

When setting the background, add a themed background with rounded corners.
No longer setting rounded corners on the layer, since the focus ring get
cropped.

Bug: b:329638387
Demo: http://b/329638387#comment5
Test: Ran ash unit test and manually tested on DUT
Change-Id: Ib071840eb175ae9cf65e10a7a4d9a84f46005f7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5436273


Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Prameet Shah <phshah@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283983}