WallpaperManager clear API changes

The goal is to align WallpaperManager#clear with
WallpaperManager#clearWallpaper.

There are two changes:
 - clear(FLAG_SYSTEM) will no longer clear both wallpapers, it will only
 clear the FLAG_SYSTEM wallpaper (and migrate it to the lock screen if it
 is a shared wallpaper)

 - clear() and clear(which) will always set the default wallpaper
 defined for the device. They won't set an ImageWallpaper unless it's the
 default wallpaper.

Bug: 295176688
Test: atest WallpaperManagerTest

Change-Id: I48a83b13eed9a5035f00405f679baf0163bd217d