I've just encountered an link-only answer. What should I do now as a good Android Enthusiasts citizen?
Do link-only answers justify a 'low quality answer' mod flag?
Yes, because the flag originated with a good intention. The mods have the ability to convert the answer to a comment and should do so.
No, the flag should be declined. The answer is salvageable by editing, there is no mod intervention needed. The user should have been simply downvoting the answer and leaving a comment like
We expect answers on Stack Exchange to be self-contained. At the very least, should that link no longer work this answer becomes useless. At the very least, please summarize the information to be found there (don't just copy-paste) and use the link as a source.
As to my experience, most link-only answers I've seen are made by some user12345 with a rep of 1 and no other posts. He either does not return anyway, or simply uses user23456 next time if simply downvoted. So no benefit here from downvoting. In this case, I can think of only two possible actions:
If the user has a rep of, say, 200+, downvoting will probably cause an update of the post, as he's much concerned about his rep. So in this case I'd recommend the "downvote + comment" method from Flow's "No" answer.