Fix touchable region calculation in NavigationBarController

This is a follow up CL to my previous CL [1], which aimed to
automatically adjust InputMethodService.Insets so that the tap events
on the navigation bar region can be sent to the IME.

What I forgot was that View#get{Left,Top,Right,Bottom}() returns
values in the window local coordinates, not in the screen coordinates.
As a result, in some cases the keyboard area could become untouchable
unless the IME specified

  ViewTreeObserver.InternalInsetsInfo.TOUCHABLE_INSETS_REGION

to

  InputMethodService.Insets#touchableInsets.

With this CL, touchable region should be correctly calculated.

 [1]: I3e7e1f83554444131e2765dc159617bb9e2337c7
      ff7b453c

Fix: 226566506
Test: manually verified with the IME mentioned in the bug.
Change-Id: I0fe54efac80dd0d55f4ba37cfa7d7188b642abb0