Guard network notification callback by check if state was changed

Existed solution was to noisy and triggered a lot of XMPP pings.

Change-Id: I6d95d5f7323e9a15fcc088b766ba903d83bccbb1
Reviewed-on: https://weave-review.googlesource.com/1381
Reviewed-by: Vitaly Buka <vitalybuka@google.com>