Don't set datetime when auto time sync is enabled

This became more visible because we were calling timedate1_call at startup, on widget creation the signals were getting emitted.

By moving the g_signal_connect_* calls into the class' init we avoid the startup activation.

Let's also not call timedate_1_call to set the time when Network time synchronization is active, because this call will fail with:

GDBus.Error:org.freedesktop.timedate1.AutomaticTimeSyncEnabled: Automatic time synchronization is enabled

Fixes #2945 (closed)