Fix assertions when ImageWallpaper is not default

This is a partial cherry-pick (see "Merged-In").

After a call to WallpaperManager#clear, don't assert that
getWallpaperInfo() returns null; instead, assert that getWallpaperInfo()
returns either null or returns a WallpaperInfo object for the default wallpaper

Bug: 307898569
Test: treehugger (check results for WallpaperManagerTest)
Change-Id: I764c014d4c2e4c1bd9cd6bbf81514019c3028499
Merged-In: Id4d65a5a808ee246a4e851e8212eb0207404f6e3
1 file changed