Telephony Data: Introduce DataAllowed state

Currently Data Connection state transition from sending allow data
to attached state skipping data allowed state. This is incorrect
as first one is configuration and second is registration with network.
Adding new state to move to DataAllowed state once configuration is
updated correctly.

Change-Id: I57f4f78759ddb74dc3da0488a5c38241edbd41a1
CRs-Fixed: 962649