Fix split->fullscreen shortcut logic

- After some debugging, the current logic works, but we close the wrong app
- Also adjust the logic to be more readable

Test: Go to splitscreen, interact with both apps, and use the split->fullscreen shortcut to go to fullscreen. Make sure the app going to fullscreen is the last interacted one.
Fixes: 309743780
Change-Id: I811595c6af8c17de47a6d113ec495d37ee325319