Ensure nav bar is not touchable when the IME is handling back

- During SUW we should calculate the full touchable region and
  not assume the entire frame is touchable if the IME reports
  handling the nav and the IME is currently visible.

Bug: 218322263
Test: Verify touchable region in SUW, SUW + IME, post-SUW,
      post-SUW+IME in both nav modes
Change-Id: I96126551112abdbee921ca7aee711b63899b6aa9