Open Source should not be restricted to a walled garden. When you appraise your app as being "open source", always consider making it available openly as well – and not just "available at Google Play".
Good options include of course #FDroid or, should your app not fully comply there, the #IzzySoftRepo – but the least you can do is tagging your releases and attach a signed release APK 😉
Oh, and of course make sure to declare a #foss license for it 😄