Bug 1066530 - tap_back_button doesn't work with new gaia-header

No, but it's useful in mtbf test which tends to run test with least cleanup. You can find in https://github.com/Mozilla-TWQA/MTBF-Driver/blob/master/mtbf_driver/tests/mtbf/keyboard/test_mtbf_email_keyboard.py
The function was called back_to_main_screen()