The opaque navigation bar guard view does not make much sense when the
IME does not show software keyboard at all.  LatinIME does not show
any UI when the hardware keyboard is connected.

With Iea77915ecc55eedaf19899e72c44f704ba9d852c, input method can change
the navigation bar visibility.  This CL changes navigation bar invisible
when the hardware keyboard is connected.

Bug:22564251
Change-Id: I14d9490e00caa852035a05830e76114cbe6af8f2