Vertically center the landscape navigation bar.

We used the paddingTop parameter in the rot90 layout to
pretend that there was a status bar shifting all the
navigation icons down; by setting this to 0 we now spread
the navigation icons evenly across the entire display.

Bug: 5530481
Bug: 5459659
Change-Id: I7f8663049c597300818549f18d7a419537f44b96