Check apk restore before restoring data

Only restore data if the apk restore succeeded, but make an exception
for the special backups which do not have apks.
Find ownership of special backups

User and group ids need to be found before copying the files of the
special backups. This is different from regular backups and the old
method to set ownership and permissions didn't work correctly since it
didn't know the original ids of the files.
Remove code to read old log files

Backups done before version 0.2.0 should be redone before upgrading to a
version beyond this commit.
Only log path to apk when backed up

Because the filename of the apk can change, the one in the log needs to
correspond to the file actually backed up. The value for sourceDir which
is stored in AppInfo corresponds to the path to the currently installed
apk.