Center GettingStarted modal in IE7+

* Rearrange the elements created in the showCTA method so that they
  can be styled with CSS tables
* Use a fixed position container in IE7+
* Initially center the modal with an absolutely positioned container
  in IE6

Bug: 55487
Bug: 61055
Change-Id: I844a6126ec544e2818e9203b3e8c38ec3a124d3b
Merge "Center GettingStarted modal in IE7+"