Prevent position topmost task to top

Home stack was reordered to bottom after user switched. So, we
were able to navigate through all tasks by back key.

Position topmost task to top doesn't seem necessary, but had
unintentionally moved its parent to top.

Bug: 148087585
Test: switch user and go back
Change-Id: If0230a7ef5a118b442bea72e7a35defe9d4dea75