Fix not able to launch activity adjacently

Make it considers launch flags when finding launch root for existing
activity and add back set launch adjacent flag root task logic which was
accidently removed by Ie259abc1.

Fix: 204578587
Test: launch new/existing activity with launch-adjacent-flag will launch
      into split root and trigger split screen.
Test: passed existing tests
Change-Id: I0e637c198de9c7b08e27864771cebd951e4fdac2