Use the display of existing task as preferred

While reparenting a task to an external freeform display,
the task was moving back on default display because of
no preferred display information specified - option and
source are null for #getPreferredLaunchDisplay - while
laying out the task.

For example, opening Chrome on a secondary freefrom display
will only bring back Chrome on default display.

So, we use the display where the task was on as the preferred
display when nothing specified.

Bug: 118354537
Test: verified by following the reproduce steps in comment#1
      atest TaskLaunchParamsModifierTests

Change-Id: I6d7a9e2d53a69bd2675dbab812a9e1a7791b1342