Don't assocaite starting window to task for overlay TF

Before, we always associate starting window to task for embedded TF.
Now we don't do that for overlay TF because there must be a visible
activity in a task, and we just need to show splash screen on the
overlay TF.

Test: manual - demo app
Test: atest WmTests:ActivityRecordTests
Fixes: 321881164
Flag: EXEMPT bugfix
Change-Id: Ib361b45aa3ccd71f09394b19ab38939fbf9015b9