Check hasNavigationBar when showing IME nav bar

Before my CL [1] the IME navigation bar was directly tied to the System
navigation bar, and would follow its visibility.

Now that these two are untied, the IME navigation bar must check whether
the System navigation bar will be drawn, to avoid showing itself when it
shouldn't (e.g. emulator).

Test: run on emulator, check that IME navigation bar is not shown;
  atest InputMethodServiceTest#testNoNavigationBar_thenImeNoCaptionBar
Bug: 296435448
Change-Id: Ide89075836a527d806f28cbeeb8d90334b79428f