Fix issue with global drag from taskbar being disabled

- BackgroundAppState extends OverviewState, so in some cases we will
  transition to background app, which inadvertently triggers global
  drag and drop from being enabled

Bug: 269819279
Test: Enter split, cycle the screen and try to drag an app from the
      transient bar
Change-Id: I0e8c5e8bbaa0220fc870d2db866152864253010f