Update the Mobile tests to work with Firefox Accounts and Appium

This is for https://github.com/mozilla/marketplace-tests/issues/500

Pytest-mozwebqa needs to be updated first : https://github.com/mozilla/pytest-mozwebqa/pull/127

Also, new account is needed because the old one was blocked by inactivity by AOL https://github.com/mozilla/webqa-credentials/pull/175
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I need to change this when https://github.com/mozilla/pytest-mozwebqa/pull/127 is merged