Init nav mode in TaskbarActivityContext between fold/unfold

In RotationButtonController, we show the floating rotation button either when the bar is visible, or when the device is in gesture nav mode, even in an immersive app. The issue here is that mNavMode is not correctly initialized after the device is unfolded.

Fixes: 272164624
Test: Open townhall. Fold and then unfold. Tilt the device and make sure the floating rotation button shows.
Change-Id: I09025132fdca5c764e66d9ac21f29b853741d257