Only save launch params for leaf tasks.

It doesn't happen in R because back then ActivityStack inherits from
Task and we pass ActivityInfo to it when it's created so setIntent() is
called and Task#realActivity isn't null.

Bug: 178789502
Test: System won't crash when setting display windowing mode.
Test: atest TaskRecordTests with no new failures.
Change-Id: I29adf18700a85593a80d08cf5be998542a97fc51