Only use nearest touch frame for button nav

- When calculating the touchable region for the nav bar window
  in gestural + IME, we can not use the cached screen space nearest
  bounds in particular because the region needs to be in window
  space, but also because the IME may want to handle the area
  under the spacebar

Bug: 180881900
Test: Open IME, rotate a few times, and try to dismiss
Change-Id: Ib02daa22d327b51953d2107ae9fd498561ee0272