Use size to inflate nav bar buttons instead of rotation

For a landscape or close-to-square device, the button layout of
nav bar was abnormal since System UI assumes the nav bar position
would change if device rotated.
However, for default landscape device, nav bar will be always on
the buttom.

This patch resolve the assumption by using nav bar
direction (horizon/vertical) instead of rotation.
Also renaming the layout for consistency.

Test: atest SystemUITests
Test: atest NavigationBarFlaterViewTest
Test: manual - rotate the phone and comapre to my Pixel2
Test: manual - test the peproduce steps and the layout of nav bar buttons is normal
Fix: 124428114

Change-Id: I23444bcb2a44555e7116f6140a29147c5a03ed69