Mozbench android

Enabling mozbench on Android. 
Main features:
* **AndroidRunner** to manage running benchmarks on Android
* **install_fennec** to handle Fennec installation
* **--use-android** argument to specify Android benchmarking and hold device id

I think we need to return 'None' here if there is an error and add a return True to the bottom of this function to be consistent enough with 'install_app' for errors to stop the benchmarks from running.