The most recent Android guidelines define clear patterns you should follow, both for ancestral and temporal navigation.
In your simple example, you should just use the default back button, since the ancestral and temporal structures are the same.
For more complex use cases, you should keep in mind that in Android there is a distinction between BACK and UP behavior.
BACK is what you get when you click the default back button. You don't need to implement it, it's already there. BACK serves a dual purpose:
- Historical back when navigating between screens
- Undo the last thing that happened. For instance, when you click a form text field, the touch keyboard comes up; hitting back will hide it again. Another example: when an alert Dialog appears, hitting back will cancel it.
It is therefore targeted at temporal navigation.
UP should be used when your navigation hierarchy doesn't necessarily match with the temporal flow. This is very common when a you have several sibling screens in your hierarchy. Suppose an online shop app with an order confirmation screen. From this screen, you can open details for each product in your current order. You navigate from the order confirmation screen, to the first item, than to second, and so on. When you reach the last item, you probably don't want to go back though all the products again until you reach the order details screen. Here you need UP.
This is ancestral navigation and the way to implement is in the action bar, new in 4.0. You can safely ignore the traditional patterns that were used in Android before 4.0, because virtually everyone is adapting to the new ways, starting with all Google apps. For pre-4.0 compatibility, Android provides a support library (you can download it using the SDK manager).
You can find further details on Google's Developer pages, namely http://developer.android.com/training/design-navigation/ancestral-temporal.html and http://developer.android.com/design/patterns/navigation.html.