Fix bug with trying to split an app with itself

This patch fixes the following user flow:
1) App is already running
2) User initiates splitscreen from Home with that app
3) User selects the same app from Taskbar or AllApps

Previously, this caused a crash because the split-from-home initiation removed the corresponding app tile, causing a null pointer exception when the same task ID was used as a split target.

Fixed by adding a null check: if the target TaskView can't be found for any reason, fall back to launching the second app via Intent instead. If the app doesn't support multi-instance, the UI will now show an attempted split, followed by the message "This app can only be opened in 1 window."

Fixes: 263041522
Fixes: 266218404
Test: Manual
Change-Id: I39ed60c9ac758ac215391f0618f44f7fcee4f32c