Defines flag for ravemped WPP UI (1/5).

This is a cherrypick of ag/20752492

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: I7032669d4a954465485a570874f5a964e5e819da