Do not reset preferred top focusable stack before positioning

Top resumed activity was not updated when starting home activity
while home was already resumed and had a pinned stack on top.

When starting home activity, the home task was moving to top,
the preferred top focusable stack was reset before positioning
home task. So, the top focusable stack was the pinned task
because there had a focusable activity (pip-menu activity)
on pinned task.

Bug: 159616359
Bug: 156314330
Test: atest PinnedStackTests#testFinishPipActivityWithTaskOverlay
Change-Id: I44313d1fe9e3d1e7a7a907b9b3eff920032198a8