timesyncd: only listen to clock changes when connected

timesyncd: only listen to clock changes when connected

This reverts previous commit and applies a different fix.

manager_clock_watch() callback calls manager_send_request() to kick
off a resync. We can only do that when we're actually connected to
something. It is not useful to setup the callback from manager_new().

Now the callback will be dropped in manager_connect() and requested
in manager_begin().

https://bugs.freedesktop.org/show_bug.cgi?id=80932