What to use?

• appium - write it in JavaScript - http://appium.io/
• directly UIAutomator - write it in Java/Kotlin - https://developer.android.com/training/testing/ui-testing/uiautomator-testing.html

Special requirements

It should handle testing across application boundaries. In Android that would mean testing activity that starts some other activity from different app for result. Like browser or gallery photo picker.