applets/taskmanager: Always set mprisSourceName to parentPid
Conditionally setting mprisSourceName to either AppPid or pidParent made no sense, because:
- AppPid was not defined in this file so it didn't work
- pidParent was set to the value of AppPid anyway, rendering the two branches of the conditional identical even if it did work
Just set it to pidParent directly. This fixes a console error.