100% unit test coverage for FileJournal and NullFileJournal. 100%
integration test coverage for DBFileJournal. Unit tests for
DBFileJournal once it supports injection.
I removed FileJournal and NullFileJournal from the list of classes that
FileBackendTest tests. It doesn't actually test them, it just happens to
run code from them without checking its correctness at all.
Depends-On: Ic22075bb5e81b7c2c4c1b8647547aa55306a10a7
Change-Id: I46d10ab7b87c23937aa04d7ec1922abfcf3bd611
