ZooniverseClient: Avoid an UnsupportedOperationException from volley.

Don't call Volley.newRequestQueue() if it would cause
an UnsupportedOperationException from the MockContext.getPackageName()
as used by ProviderTestCase2. Luckily our content provider test case
doesn't actually need to do any network IO.
ItemsContentProvider: Handle Context.getExternalCacheDir() failure in tests.

Now all the tests complete.