Ensure we reset reparenting when launching new splits

- Follow up to ag/24812535, there were a few cases where we don't call
  into prepareEnterSplit() but still need to reset the reparenting
  state to prevent trampolines from breaking splitscreen upon entering.

Fixes: 292454704
Test: Launch split from the menu for app that launches a trampoline
Change-Id: Ia0d825192c3ec78991a75f0501e7ece9d75cb4af