Center navigation icons for car nav bar.

The new HVAC design calls for the temperature buttons to be on the
side of the nav bar, at the same position as the nav icons. Decreasing
the width of the button container to 760dp as per go/aae-ncar-por.

Change-Id: Ic2878be0d42cf37ba09bb3aeec68813ad3d1fb8a
(cherry picked from commit 3431cdac24758e7d29cd0e1878f4b6349d2a7950)