Add null-check to findLastActiveTaskAndRunCallback

ItemInfo.getTargetComponent is nullable, so we can't always create a ComponentKey. Added a null-check and proper creation of ComponentKeys

Flag: not needed
Fixes: 286053950
Test: started and completed splitscreen selection from home, taskbar and overview
Change-Id: Ifa30f194ae064fab8aad79c5116f8c859dfd8cf1