Define flag for wallpaper preview loading animation (2/3)

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: I4fa8e2bf238798be4f2ad87cf83fc65d78091e19