Fixes unable to start recents activity

Another home activity was on top of the Recents activity in a separate
task, but both were in the home root task. The Recents activity did not
launched behind when swipes up in gesture nav because the home root task
was already the top-most root task.

Home Root Task
   - home task #1 (Other home activity)
   - home task #2 (Recents activity)

Start the Recents activity if the belonging leaf task is not the top-most.

Bug: 184241073
Test: test gesture nav with the sample app
Change-Id: Id1f0d4d81a0246d6814e482d25e49c36d89fb55d