Get candidate stack for launch if possible

An activity stack was created while starting an activity in split-screen
mode. Since the activity did not landed on the stack eventually, the empty
stack leaks after activity launched.

We should get target stack from the candidate task in the first place
if possible.

Also make sure empty stack is removed.

Bug: 142345211
Test: atest ActivityLifecycleSplitScreenTests
Change-Id: I366a5690b26c4c564cf43f9057408f543a76a7fd