Amazon is putting this on iOS first because that's where they're likely to make the most money from Amazon Prime subscriptions and video content, seeing as it has such a large userbase (of compatible devices).
The general non-Fire Android version will come next. They really have no reason not to now, since it's just a gateway to making more money. I think it's period of being an exclusive and Fire-seller is coming to a close, and they'll open it up. There will probably be some new exclusives/benefits to owning a Fire 2 (or whatever it is) to heavy Amazon users that won't be on other platforms.
Hopefully we'll be watching our stuff on our N7's without FF/Flash soon.