Fix result code of launching existing activity in split

Since all secondary leaf tasks will be reparented to the organizer
created task, when launching an existing task in background in
split-screen mode, the root task mTargetStack is always the
organizer created task which is always the top of display area.
Then mMovedToFront is always set to false.

This may cause missing some launch metrics in split-screen because
ActivityMetricsLogger won't count the number for result code
START_DELIVERED_TO_TOP.

The test testSplitScreenTaskToFront didn't catch the problem
because it still creates tasks in the old way (multiple split
secondary root tasks under display area) which doesn't exist
in current design.

Fixes: 176277089
Test: ActivityStarterTests#testSplitScreenDeliverToTop
      ActivityStarterTests#testSplitScreenTaskToFront

Change-Id: I6bdb1faf8f7dbfb1e8a6f75279b87ffc9072fb31