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
1 file changed