Initialize navigation mode for TaskbarDelegate

* Previously we were only initializing nav mode for
EdgeBackGestureHandler and never updating state in
TaskbarDelegate

Fixes: 260226345
Test: Wrote new tests,
killed sysui process and dump state in terminal and
observe nav mode matches between NavigationModeController
and TaskbarDelegate

Change-Id: I637fbf759ab86cd9ff25875e24dca9327fee35fa