applets/taskmanager: Always set mprisSourceName to parentPid

Conditionally setting mprisSourceName to either AppPid or pidParent made no sense, because:

1. AppPid was not defined in this file so it didn't work
2. 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.