Update status.ObservedGeneration for sources resources when sources resources get reconciled

[capri-xiyue]

In PR knative/eventing#1849, we tried to validate dependency by checking whether the ObjectMeta.generation of dependency is equal to the Status.ObservedGeneration of dependency.
However, we found we don't update status.ObservedGeneration for sources resources when sources resources get reconciled successfully. We should do that when sources resources get reconciled successfully.
The sources resources in the knative-gcp repo is "pullsubscriptions" and "storages"

[capri-xiyue]

/assign @capri-xiyue