Create an alpha letter scroller

- The function used is a raised cosine function to determine the sections to animate.

Tested flag: adb shell device_config put launcher com.android.launcher3.letter_fast_scroller true
Based on recent figmaspec, size of selected letter is 28dp and regular is 20dp. That means scaling should be 28/20.
Colors are:
onSurface for letter
surfaceContainer for background.
Figma shows selected colors are materialColorOnSecondary

bug:358673724
Test manually: https://drive.google.com/file/d/1gvJd5KoK8X_Sv8jXg1cZ65R4AuqHuvcd/view?usp=sharing
Flag: com.android.launcher3.letter_fast_scroller

Change-Id: Ia86eb0e8c41bc043fda3c44aeaf59e204429f337