Bug 942220 - Add explicit wait in tap_back in contact_details.py

I added a wait before tapping the back button because I ran <code>test_edit_contact.py</code> many times and I noticed that sometimes that button is pressed really quick, so we can't actually see the moment when it does it. Instead of <code>self.wait_for_element_not_displayed(*self._back_button_locator)</code> I could add <code>self.wait_for_condition(lambda m: m.find_element(*self._back_button_locator).location['x'] == 320)</code>
Let me know which option you think is the best. Thanks!