Bolster the wait before tapping the add_bookmark_to_home_screen_choice_button

This addresses the recent failure as seen at [1]. Although this isn't a common failure this wait should help prevent it from happening again.

[1] http://qa-selenium.mv.mozilla.com:8080/view/B2G/job/b2g.unagi.gaia.nightly.ui/298/testReport/junit/%28root%29/TestBrowserBookmark/test_browser_bookmark/?