Move WallpaperData in its own class

This CL was entirely made by IntelliJ's refactor tool, except the field
WallpaperData#callbacks which has been moved from private to default visibility.

Note: the linter complains that the WallpaperData fields should start
with "m". We may address this later, but for now renaming these would create
unneccessarily more risks of conflicts.

Bug: 264637309
Test: atest CtsWallpaperTestCases
Test: treehugger
Change-Id: I91c79a6988fada04886632844822de02e5dfde7a