Don't query user ID via main thread binder calls

This removes constant binder calls for User ID on main thread from
NavigationModeController - cached value in UserTracker is used instead.

This also uses UserTracker for user change callback to ensure timing
consistency (previous approach uses the same signal as well).

Bug: 283808194
Test: since there's no automated tests, manually tested nav mode
      controller while switching user profiles on cheetah.
      Monitored both logcat and product behaviour to check that
      user ids still correctly change.
Change-Id: I89ad7711282a31286b1f7b9092098817bb0f2d01