Fix bug 4065021 - Adjust spacing of the "home" action bar item to be

consistent with the "up" indicator

Make sure that the "home" affordance in the action bar always occupies
the same amount of space regardless of whether or not the "up"
indicator is visible. This means centering the application icon/logo
in the remaining space after the "up" indicator has been added.

Change-Id: I3c81cfe8255546d4dd676af913895713baba4f13