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.

The change also includes a crash fix from nemomobile tree:

https://github.com/nemomobile-packages/ofono

Commit: 2329468e251f98d0f23b503a7860a8792c0031c8
Author: Martti Piirainen <martti.piirainen@oss.tieto.com>  2014-05-15 06:32:58

    [nettime] Prevent crash if MNC or MCC is missing
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