The Android emulator is hot-deployable. Once you save and click 'run' (assuming no compile errors) it will package and re-deploy to the emulator which will then restart the app to run the new version. The same is true if you have an Android Developer Phone connected via USB.
If you get the message “Warning: Activity not started, its current task has been brought to the front”, it helps to quit/move from the front the running app in the emulator by pressing the back button. Seems like Android does not overwrite the running app in this case.