Long-tap to see tooltips is a convention that has been proposed in various incarnations of the Android UI guidelines. The convention was present in very early Android UI guidelines (e.g. Android 4.x); I'm not sure if it still is.

The problem is that long-tap gets used for a lot of other things on Android (extended selection, particularly). The user experience is pretty unhappy if you use both conventions. And, in practice, long-tap tooltips are so rarely provided on Android apps that I doubt that ordinary users would expect them at all.

As far as I know, tap-and-hold (as distinguished from long-tap) isn't a conventional gesture on Android.

At least that’s how you get tooltips on Android. Of course, tooltips were designed to be discovered by a user hesitating with their mouse over an UI element (sometimes works, sometimes doesn’t), while tap-and-hold on buttons you have to know about, and it’s still scary when the button looks like it could do something potentially destructive and you want to find out.