Use test specific build dir when needed

If a FixtureFactory was created with a nil buildDirSupplier then this
change will cause it to create a test specific directory instead. This
will allow packages whose tests have been fully converted to the test
fixture model to remove the need for the package level buildDir
variable.

Bug: 182885307
Test: m nothing
Change-Id: Ifa70acadbd90356fadbe39675bac3214d925aa2f