Avoid setting the Agent online if it needs network but there is none.

Without this patch an agent is online initially and only reacts to subsequent,
online status changes. This cause for example the imap resource to go online
when started without network connection (resulting in a wrong initial status),
and as the imap resource tried to connect and failed, it set itself to offline,
resulting in it not reacting to the network becoming available.

With this patch all network detection code can be removed from the imap resource
and it properly reacts to network changes.

BUG: 329857
REVIEW: 117008