Don‘t consider adjacent launch root if the existing task is't on top

The adjacent task is considered as a launch root task if an existing
task is already in the adjacent task of the launch target. This CL
updates the policy while the task is located in an adjacent task and
the existing task is on the top.

Bug: 280392203
Test: atest ActivityStarterTests
Test: split(A|B)->split(A,C|B),drag A to B’side, Result:split(C|A)
Change-Id: I006de0438c572defe4f770d29378c8ddfadc351d