[url_launcher] launch should return true on success.

[chingjun]

Description

The comment in the interface states that launch should return true when launching the url is successful. The current code used to return null when forceSafariVC = true.

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No Dart code change, and there wasn't any test for native code.

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[stuartmorgan-g]

#3475 ended up being this exact fix, and just landed. I'm working on fixing PR triage so this kind of thing stops happening 🙁