Skip evicting children in certain non-drop cases as well

- When splitting with an existing split pair task from an app icon
  menu we also need to avoid evicting the children to prevent the same
  race between start activity and eviction in ag/25565608

Fixes: 309905087
Test: Launch a split pair, Open a new fullscreen app, Use the taskbar
      to split with one of the previously paired tasks
Change-Id: I76808dfad48951d443593c6d3d284827ba7f2e00