Use display windowing mode if undefined

A undefined windowing mode prevented from reusing the
candidate task while cold start an activity into
split-screen from Recents.

Use windowing mode from its parent for always-create-stack
check if undefined.

Bug: 152408408
Test: atest TaskDisplayAreaTests
Test: cold start to enter split-screen
Change-Id: Ie0f5b69c9d09ad517b749827dbf2a5e7297b1075