In your variation, only the first is correct, interpretable and likely to be heard that way, no matter how formal or informal the situation.
Moving the finite verb to the end of a subordinate clause is not a matter of style or preference, it is deeply ingrained into the sub-conscious language centre of a German speaker’s brain. It is something that will throw them off if it doesn’t happen. Thus, the version with heißt at the end must be a relative clause, the version with heißt at the beginning must be a combination of two main clauses separated by a comma rather than a full stop.
Can we interpret your second sentence as two main clauses? Yes, we could. But unfortunately, they don’t make sense being that close together. To connect two main clauses by a comma rather than a full stop requires them to be similar enough. However, the first sentence is clearly a question (verb first). The second is not (verb second). But the second sentence carries the question mark. There are so many contradictions that it just won’t work.
We can change your sentence mildly, though, to allow for both versions:
Du hast eine App, die Google Maps heißt.
Du hast eine App, die heißt Google Maps.
Again, the first version with a relative clause is always okay and typically preferred. But the second version is no longer wrong, it is just ‘chunky’. It’s two loosely connected sentences, connected more strongly than with a full stop.
The second version would still, however, mostly be confined to informal and spoken German, and is more likely to occur if you are actually dealing with a chain of sentences:
Wie finde ich denn den Weg zu deiner Wohnung?
Du hast eine App, die heißt Google Maps, die machst du auf, da gibst du die Adresse ein, dann klickst du auf Weg berechnen und dann musst du es nur noch schaffen, nicht falsch abzubiegen.