Tap on the bottom of the Next button when setting up email account to address intermittent failure

This fixes the failure seen at [1]. It addresses one of the two failures discussed in issue #917.

[1] http://qa-selenium.mv.mozilla.com:8080/view/B2G%20Inari/job/b2g.inari.gaia.v1.0.1.ui.ril/82/testReport/%28root%29/TestSetupGmail/