Before, we may reuse the existing TaskFragment for secondary activity, which may be below the primary TaskFragment. Now, we make sure the secondary TaskFragment is always on top. Bug: 244254584 Test: atest WMJetpackUnitTests:SplitPresenterTest Test: atest WMJetpackUnitTests:TaskFragmentContainerTest Change-Id: Id4c240af71fbd344126ae8bbbad57d9c2150d550