SurfaceFlinger: Correct DetachChildren case in SFFakeHwc_test

According to commit 161410b0, layers
with the same client as their parent should not be detached. Thus, the
test case should be modified to have DetachChildrenSameClient and
DetachChildrenDifferentClient two test cases

Test: Run sffakehwc_test
Change-Id: I07f4d4ab2dd2024bfa2de2dfa68e28c468066252