resourceloader: Update MessageBlobStore documentation and code cleanup
* Update and improve class documentation.
* Remove comment that claims to return false if the module has no message.
generateMessageBlob() returns '{}' in that case. And this is expected since
we want to cache the absence of messages.
* Re-use the module objects passed to MessageBlobStore::get() in getFromDB()
instead of using ResourceLoader::getModule(). The object is the same either
way (since ResourceLoader re-uses the objects also) but reduces complexity
of getFromDB() to not need the ResourceLoader object as parameter.
Change-Id: I89a14d7185877fae52791f6837883ed3a6749cd7