You are absolutely right, Unity application requires heavy processing power.
You can develop native Android application just using Android Studio. There are many SDK including ARToolkit(Open source) and other including Easy AR provide Android lib
Here is advantage of Easy AR which provides Java as well as C++ code
But in following cases Unity3D is more useful:
- Cross platform app dev
- 3D Models and interactions are more in application
- Need Particle effects like fire etc
- Object based rendering and animation