Use config_imeDrawsImeNavBar in Launcher

With this CL, the Launcher starts checking

  config_imeDrawsImeNavBar [1]

instead of

  config_navBarInteractionMode

when determining whether the IME should render the back and IME
switcher buttons or not.

There should be no observable behavior change as long as

  config_navBarInteractionMode

is set to true when the gestural navigation is enabled.

Bug: 216118048
Test: presubmit
Change-Id: Ia8269e240e0b18e1c7b8bd550dbb197ad58a6dbc