[timed] Enable connman NTP time synch when NITZ time management is enabled

Currently, the automatic time update is based only on the NITZ signal [1] that timed receives via ofono from the mobile network. Not all carriers support this, though.

Connman can be set to maintain time from NTP via D-bus [2]. This PR makes timed enable the NTP time adjustment feature of connman when automatic time updates is enabled.

[1] http://en.wikipedia.org/wiki/NITZ
[2] http://git.kernel.org/cgit/network/connman/connman.git/tree/doc/clock-api.txt?id=HEAD