Fix drag to split gesture puts an existing task into pip mode

And flag drag-to-split launch with no-user-action to prevent sending
user leaving event to the current top activity since it's going to be
put into another side of the split. This prevents the current top
activity from going into pip mode with user leaving event.

Bug: 223936167
Test: atest WMShellUnitTests
Test: Drag an app icon onto a task which supports auto-pip, it'll enter
      multi-window mode instead of pip mode properly.
Change-Id: Ifbeccbd19c8c83a31e116d9a73a2d7432c07917f