Back up wallpapers via staged copies, not hard links

File-based encryption regimes turn out to make the zero-copy approach
unworkable.  We now copy the file to the local stage for backup purposes,
making sure only to refresh the copy when it changes.

Bug 30201058

Change-Id: Ie3f29ed63f778e3e0e00d1cf56b0bb37553b7823
1 file changed