Wait for the IME nav bar to become visible

This attempts to fix an issue with unit tests not finding the IME back
button and IME switcher button, by first checking that the IME
navigation bar becomes visible, as we are in gesture nav mode.

Flag: EXEMPT testfix
Bug: 395783236
Test: atest InputMethodServiceTest#testBackButtonClick
  InputMethodServiceTest#testBackButtonLongClick
  InputMethodServiceTest#testImeSwitchButtonClick
  InputMethodServiceTest#testImeSwitchButtonLongClick
Change-Id: I5ae6fc07600cfe59bcf65f48f3e347d71f34db75