Always inset by nav bar frame when IME is visible

- When the IME is floating and does not report insets, the app content
  is never inset to accomodate the visible dismiss-ime button in the
  nav bar.

Bug: 142218188
Test: Enable floating ime (no reported insets) and verify that the
      app does not render under the nav bar

Change-Id: I3b8e03a4e9cf82b12c2054db479a8a027bc9c918