Keep ime information up-to-date even when divider invisible

Otherwise, it can accidentally put the divider into ime
mode when there isn't an ime.

Bug: 159380790
Test: with minimized primary, open ime in launcher then expand
      primary to leave split mode. Re-enter min primary and
      rotate device. Open an app in secondary and make sure
      it doesn't dim the primary.
Change-Id: I68a9b56073b247e0a63c6670a9e1b330014bebc8