Mark header bar buttons as clickable only when they are made visible.

- We were incorrectly setting the OnClickListener to null to try to do 
  this, which still causes the touch event to be consumed by the button
  view, so instead we should just set the clickable state directly.

Bug: 27859552
Change-Id: Ida3b4a467c6f325ce8fe5a90b0b77c2bdda5a936