Mark wallpaper as backupInForeground="true"

It's fine for wallpaper backups to proceed even while the system uid
is considered to be doing something foreground-equivalent, since it's
in its own process and killing it won't interrupt the actual system
work.

Bug 30662562

Change-Id: I463c1ed221da17fbeb336b3be09d3b1ac47aca80

packages/WallpaperBackup/AndroidManifest.xml

@@ -24,6 +24,7 @@
                  android:process="system"
                  android:killAfterRestore="false"
                  android:allowBackup="true"
+                 android:backupInForeground="true"
                  android:backupAgent=".WallpaperBackupAgent"
                  android:fullBackupOnly="true" >
     </application>