Make sure the secondary TaskFragment is above the primary

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