Only lookup dataLoader when necessary.

This avoid the race condition when we are about to seal the session, but
another notification goes through and fails the commit.
Also saves some resources - we almost never need a dataLoader for e.g.
Incremental installations.

Bug: 188079906
Bug: 187967058
Fixes: 188079906
Fixes: 187967058
Test: atest PackageManagerShellCommandIncrementalTest
Change-Id: Id931b40362f603ccd45b64ff5f8636d8c32e849e