Enable the split-items from the overview panel

Consider the undefined activity type to be dockable since the task of
activityType is undefined before root activity adds to the task.

Bug: 239388308
Test: 1. Have tasks in overview.
      2. Reboot the device.
      3. Check split-items enabled from overview panel.
Change-Id: I5741022d9065243bc2cbc6a8ba8b2d3c2883936c