You are unaware of monopoly law. In the US, the legal threshold for monopoly power is generally around 70%, though cases have been brought with as little as 51% share. In the EU, the threshold is 40%, and in the UK it is 25%.
Furthermore, the actual figure for Europe is 63% Android, more than sufficient to meet the monopoly threshold test.
A Samsung Galaxy can run any OS designed for that device. And Adroid now runs not only phones, but tablets, smart watches, Smart TVs and automaker's media centers. But all this is irrelevant. The legal test for market share depends on how much two products compete with each other, not whether they interoperate or not.