nettime: Fix it for multi-modem setups

Previously, ofono was storing network time in the same place when more
than one modem was present in the system. Also, we copy now MCC and MNC
as the current operator may have been removed when querying the time.
doc: Add org.ofono.NetworkTime interface

Based on initial documentation for nettime plugin from Nokia:

https://lists.ofono.org/pipermail/ofono/2011-January/007441.html