Defines flag for ravemped WPP UI (4/5).

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: Ia3f85b4ecd63a70d48a8cd26a88a7d93f17f44db