Defines flag for ravemped WPP UI (2/5).

This flag is owned in System UI and served to Wallpaper Picker (WPP)
through the existing content provider for quick affordances.

Bug: 262780448
Test: manually made sure that I can read the flag value in wallpaper
picker and its value matches whatever I set the flag to.

Change-Id: I26235a0718cc44c343713a5aef75750f4e59cfa7