phosh: Indicate lack of connectivity

We show no icon if we're connected but indicate the lack of a connection (or if we have a connection and can't reach the internet).

This is useful to identify if e.g. when a mobile connection but mobile data is not activated or if one is stuck behind a cative portal.

The icon is only added to the top bar since the information is not very valuable on the lock screen.

Closes: #229 (closed)

Signed-off-by: Guido Günther guido.gunther@puri.sm