Rename Theme.SystemUI.Light

This theme was introduced before dark-theme, and signifies that lock
screen will have dark text, because the wallpaper is light.
A better name for it would be Theme.SystemUI.LightWallpaper

Bug: 183953523
Test: visual
Test: atest StatusBarTest
Change-Id: I036f935b6f9f61ed8192570911fdd958cb8f65e0