Sanity-check existence of restore agent

When a restore dataset includes data for an app that used to have
a backup agent, but does not in the currently-installed version, we
were merrily trying to bring up the agent for restore anyway, and
crashing.  Now we don't do that; we check whether there's actually
going to be an agent to handle the data before doing any of the
heavy work.

Bug 7130695

Change-Id: I0a38c2a8bb51d4a140a72d22896fa58d98ebaa02