Don't log an error if system wallpaper is applied to lock

Don't log an error if the lock image doesn't exist and the system image
exists, since this is an intended behaviour. If the lockscreen was
supposed to have an image but the image could not be backed up, we will
see the error from the backup side. Adapt the test accordingly.

Now, we only call either both restoreFromStage(FLAG_LOCK) and
restoreFromStage(FLAG_SYSTEM), or call only
restoreFromStage(FLAG_SYSTEM | FLAG_LOCK).

Also, use this opportunity to make sure we only call file.exists() at
most once, to reduce IO operations.

Test: atest WallpaperBackupAgentTest
Bug: 283091821

Change-Id: Ia19fcefc1676e8bd1fc0cf80c7a959c4d6f5839f