Added assertions that both apkName and repoAddress need to be populated
in order to call `getUrl()`. Also verified that this is the case for all
usages of this method, which it should be. All `Apk` objects which currently
have `getUrl()` called on them are loaded using the `ApkProvider.Helper.findById()`
method without specifying which columns to load (which defaults to all).
3a1fcfd226