SystemUI: update navigation bar layouts

For tablets, mimic stock button paddings.
Except for when 5 buttons are used, then
distribute them equally across the navbar.

Change-Id: I99ccc07d31bc7b719c44be74f164d88d7dc542d5
Signed-off-by: Roman Birg <romanbirg@gmail.com>
Merge "SystemUI: update navigation bar layouts" into kk44