Fix ZOrderingTest got NPE during display creation.

ag/5431947 modified
HierarchyRecordingBuilderFactory.make() & HierarchyRecorder.build() for
SurfaceControl creation that will set mTransaction as null after method called.

It will got NPE when creating the second SurfaceControl instance
during DisplayContent creation, remove the null assigment to fix this case.

Bug: 113800711

Test: atest FrameworksServicesTests:ZOrderingTests
Change-Id: I791046afdc2f371e37c2f7d3c36b7f2484339e08