Define flag for wallpaper preview loading animation (1/3)

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

Bug: 274443705
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: Icb9b7fe7985b16c07beac44bfda63abb5cfe1956