Make car nav bar center it's children.

The linear layout that represents the buttons in the nav bar are added
programatically. Its xml layout values are overwritten, so set the
gravity to to center in its parent to ensure the buttons are always
centered.

Change-Id: Id4d380e8010e316907bc31f0ee6bc8d4624c6667
(cherry picked from commit 288c1c41d5784ba0f5912709b3101abcd8cd7e2a)