Get the navigation bar insets frame from the window

Instead of querying the navigation bar insets source frame from the
InsetsState with the internal insets type, this CL gets the frame from
the navigation bar window directly. So it won't need to search for the
insets source in the InsetsState, and this is a step to remove ITYPEs.

bug: 234093736
Test: Enable magnification in a11y and see if nav bar is excluded.
Change-Id: I1f7b9682a2823084fc093ea95d46c1740d6b775c