Compress the preferred-app backup payloads

They can be Very Very Large, so take advantage of the new BlobBackupHelper
infrastructure to keep them tiny!

Also fix an issue in which the restore path wasn't properly passing
notification payloads along for processing, and an issue in which the
blob helper wasn't handling empty states properly.

Change-Id: I11a7ca3cd2e26f634a8971e874ac97385b0b500c