Update letter list textview background color to spec.

Spec: https://www.figma.com/design/uMzPkNMZpb7EyfHDo8usIa/V-%E2%80%A2-Toast-Butter?node-id=2746-180205&node-type=frame&m=dev

bug: 358673724
Test: manually - photo: https://drive.google.com/file/d/1RWlaSRHbXCYhsY1F3nzruldQnABszUiz/view?usp=sharing
Flag: com.android.launcher3.letter_fast_scroller
Change-Id: I03cd4fbf1e76a7ad34983ace18a69a52116e7834