Remember which apps have available restore info in the ancestral dataset

When we perform a full-system restore, remember the set of applications which
have data available in our ancestral dataset.  This is a key filter for not
having to do a round trip to the [remote] storage backend at app-install time
unless it is likely to be fruitful.

Change-Id: I7c77b490c560c581888d84f02f258b2e2d73bc69