It really depends on whether you want to develop games or standard apps.
Android Studio is the way to go if your app is structured in activities, which are basically screens. That is the case for most standard apps. For example when you open your browser on your phone, you might face a loading activity first, then a tab activity, you can possibly access a settings activity, and so on. If your app looks like that, then go for Android Studio.
Unity is a 3D-development platform. 3D is computationally intensive, especially when it involves real-time interactions, and development in this area requires you to use an already-existing engine, like Unity. Do not code a 3D game engine by yourself. If your app involves such requirements, you should use Unity, which provides you out-of-the box support for game, 3D, and real-time interactions in computationally-intensive renderings.
If your app is 2D, you can still use Unity, but there exist a larger choice of 2D game engines that you can use.