Deal gracefully with cancel of nonexistent jobs

In particular, fix an implicit assumption that jobs existing under
a given "caller" uid implies that the the same jobs must therefore
be known under their source uids as well.  This isn't true when the
jobs in question are sync jobs (caller uid = system) in secondary
users that have been torn down.

Bug: 72459151
Test: manual; automated multiuser sync testing tbd
Change-Id: I9bb6aabfabdaaee04c39236bc8e1bca696898a83