If you think people should be warned right away, Epic should have the right to notify its own users, and Google should have the basic decency to reply to a request to coordinate.
Google found the bug, they have the right to disclose it however they want. There are no legal standards anywhere that requires anyone to do anything with bug reports.
Epic doesn't have any rights here, it sucks but they're not entitled to anything. I hate this because I've experienced this from Google before (at work), I would love to have that right to hold up to 90 days before we can notify people but until there are legal standards, there is no right.
They have an ethical obligation. Security disclosures have major ethical concerns. If you're acting unethically, you are a dick.
Sure, Epic doesn't have any rights here. But we don't need a law to know people [and corporations] should not be dicks.
Google could have been more ethical by replying to the vendor after making their initial communication. They chose not to.
You are correct in that Google has an ethical obligation; and they and met it by protecting all Android users. I would say that Google making this announcement lets all Android users know that its not safe to just randomly install apps on your phone. Yes, they allow it, but at the same time the public is genuinely ignorant of any type of security practices.
This is a non-sequitur. You are equivocating between legal obligation and ethical obligation.
Google's original view of the situation was that installers like this make devices measurably less secure (to which Epic's response was pretty meh), and the installer being exploitable to install pretty much anything only proved that point and then some.
...and frankly, it would not matter who wrote the installer if it got used for a mass infection and attack. Google would still get the bad press and a generous helping of blame because of the trashy way so many "tech reporters" behave to get those mouse clicks.