[Wallpaper] Always use default wallpaper as fallback.

In today's implementation, the wallpaper manager service sometimes
fallback to image wallpaper, sometimes fallback to default wallpaper.
This causes inconsistency and unpredictable fallback behaviors based on
different fallback reasons.

This change makes the wallpaper manager service always fallback to
default wallpaper component.

Bug: 289127286
Test: manual test
Test: atest WallpaperManagerServiceTests
Change-Id: Ie2f06dba13ee6a0c96d84b9284aa8b901288deda