Focus the primary button when the CTA is shown

Give the primary and secondary buttons a tab index of 0 so that they
enter into the DOM tab flow [0] and an ARIA role of "button". Simplify
the button markup. Finally, focus the primary button when the CTA is
shown, now that the buttons are programmatically focusable.

[0] http://snook.ca/archives/accessibility_and_usability/elements_focusable_with_tabindex

Bug: 62256
Change-Id: I323c5958b512ab59107cfd56204d1fc5a7d57f7c
Merge "Focus the primary button when the CTA is shown"