Refactor WallpaperModelFactory (3/3)

This CL makes the following changes:
This CL makes the following changes:
1. Created an interface WallpaperDataFactory, which is implemented by
   DefaultWallpaperModelFactory and GooogleWallpaperModelFactory,
   respectively, depending which project we include.
   This also avoids complex inheritence pattern, where developers need
   to go back and forth to know what's going on when converting.
2. Made DefaultWallpaperModelFactory and
   GooogleWallpaperModelFactory injectable classes.
3. Moved functions in DataFactory classes to companion extension
   functions. Developers do not need to look into many classes.
4. Since DownloadableLiveWallpaperInfo represents a wallpaper that's not
   yet downloaded, we should treat it as a static wallpaper that shows
   the static image.

Test: Manually tested that the downloadable wallpaper shows
Bug: 291761856
Flag: ACONFIG com.android.wallpaper.multi_crop_preview_ui_flag DEVELOPMENT
Change-Id: Iefc7202bcd9e963cfb3cc1c5933925f87a40d663