Schedule backup of a package on a background thread

Scheduling involves writing to journal file. This part can be done on the
background thread in order to speed up the unlocking. Deferring writes to
journal file should be safe, because the system will reschedule backups in
case of a crash.

Bug: 29619732
Bug: 29645167
Change-Id: I0667ac2bf159e0f40b7a5066f41ea0841df98437