Keep nav bar position updated

The change is to update the nav bar position variable every time when we
layout the navigation bar. This is necessary to make the nav bar color
control works correctly, and also the transient bar gesture control
works correctly.

Test: Open light themed app in landscape and enable IME, check the bar
color.
Test: landscape immersive mode app swipe from the side.
Bug: 202476380

Change-Id: I736af97326763b8ccb0afb2c82bbf25edf8d25c4