[connman] Start/stop ntp activity when TimeUpdates property is changed

When manual mode was selected, ongoing ntp activity was not stopped and
system time got changed at the next ntp poll time.

Calling __connman_timeserver_sync() after the time_updates_config
variable has been changed causes ntp activity to be stopped when
manual mode is selected and restarted when auto mode is selected.

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>
Merge pull request #169 from spiiroin/timeupdates_property_change

Start/stop ntp activity when TimeUpdates property is changed