Consider adjacent launch target when recycling an existing task

If there's a launch target indicated while recycling an existing task,
consider to use the adjacent task of the launch target. So the existing
task won't be reparent to adjacent task redundantly.

Fix: 224901460
Test: atest ActivityStarterTests
Test: put single-instance app in split and make sure it gain focus,
      dropping the same app icon to another side of the split,
      observed it will switch split position without dismissing split
Change-Id: I4d4b4bcb6c036601a4fa68905ae66d91526bfe84