There's no required minimum. 15 is fine. But FYI, your app won't run on 15. The Instant Apps runtime itself isn't compatible that far back. At the moment, that only goes back to 23 (but we're working on that).
Source: I'm on the Instant Apps team. I checked the publishing validation code to be sure.
For the record, where Instant Apps is supported, minSdkVersion is respected. So, a minSdkVersion=24 app would not be delivered to a 23 device.
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