Injecting to another application requires INJECT_EVENTS permission

Hi, I need help to go back from gallery to my application!
I'm creating a test for my application (ionic 2 app) with xamarin: in my application I can go to gallery to set profile photo. In test, I need to go to gallery (even if I don't select any file) and than go back to my app: when I open gallery, the app.Back(); obviously doesn't work and I obtain that error: "System.Exception : PressKey for keycode:'KEYCODE_BACK' was unsucessful: java.lang.SecurityException: Injecting to another application requires INJECT_EVENTS permission". I found documentation about Maniferst.Permission but I can't understand how it works. Some help or examples????