Which command to use for tap/select default Back button of Android devices?

As All Android device has Back and home tab by default. So many Android app and Ours as well don't add Back button through the app  it appears as we goes to any activity at the bottom you always see back button. So as during my test case I couldn't find any command for that and I can not create Mobile artifacts for that back button as it is device object not the app object .It will be great if I will get update fro this. here I have attached screen shot about back button you see in bottom left corner.