Do not evict starting task when startShortcut

Split screen evicts the original pairing task after drag-n-drop a
shortcut into split. Launching a shortcut into the same task, it'll
dismiss the split due to one side of the split being empty.

This CL evicts the non-opening child tasks only to pevent one side of
the split being empty.

Bug: 235027308
Test: 1. Launch App1 and drag Settings' shortcut (Wi-Fi) to split
      2. Launch Settings' shortcut (Data usage) to split screen
Change-Id: I9f6ea82759a7f73f6f7f4b2617dd7b5dbb9eeb38