Infrastructure for notification backup/restore

Introduce a fairly general "store byte[] blobs as backup keys" helper, and
use that to integrate with the Notification Manager.  The helper provides all
the machinery of managing prior state, compressing the blob on the wire, etc.

Bug 19848954
Bug 20105530
Bug 17755700

Change-Id: I4aa808172244af6db0ab2f4440cc266a797655d7