The way it works with Navionics is that you download the app for free and pay an annual chart fee for one user (the amount depends on what geographical area you need) and can then use it on both Android (Sailproof) and iOS (iPad, iPhone) without extra cost. Obviously the different operating systems require separate app downloads but there’s nothing to stop you from having the app on both types of OS at the same time. You just log in to your account in the app and your current chart subscriptions are there to download.
I’ve just looked up Navionics Boating on the Google Play store for Android apps and it’s still available as a free download, with charts being extra, as above. Don’t do what I did and download the UK, French Mediterranean and Greek Ionian separately, substantially more expensive than buying the UK and the whole of the Mediterranean in one go. You used to be able to get the whole of Europe (excluding the UK) as one package but they’ve now subdivided it.