Update test so that only editable fields are interacted with.

test_user_can_update_profile_information_in_account_settings_page started failing on amo.dev because a new non-editable field was added to the user profile page.

http://selenium.qa.mtv2.mozilla.com:8080/view/All%20not%20B2G/job/amo.dev/1309/HTML_Report/

Updated the test so that it only interacts with editable input fields.