Consider new_task flag when launching with adjacent

We should consider the new_task if launching with adjacent flag,
otherwise the other split is empty. This CL ignores the flag if
there is no new-task flag present during the launching.

Bug: 234696171
Test: atest ActivityStarterTests
Change-Id: Icb646b73a24b6a9771a75f6745041d77af08f4e0