filebackend: cleaned up the FileBackend constructor

* Moved some of the graph construction work to FileBackendGroup.
  This helps the code in not depending on the rest of MW so much.
* Updated tests and FileBackendMultiwrite, which are the only things
  directly constructing FileBackend objects.

Change-Id: I188a053c70ce088ce34613d5db40e6708e3ea9b7
Merge "filebackend: cleaned up the FileBackend constructor"