Make specific nav touchable region when the IME is showing

- IMEs would like to receive touches that are near the bottom
  of the screen under the nav bar so we should allow this

Bug: 168552683
Test: Verify touchable region from dumpsys input in all nav
      modes w/ ime switcher
Change-Id: Ic2ee8cc8ce59c4025c6730195d6e1050daddf87a