Fix to allow removing sign-in notification when network removed.

When the NetworkMonitor is told a network disconnected and a sign-in
notification has been shown to the user, the NetworkMonitor requests
the notification be removed.  This request goes to the
ConnectivityService who may have already removed the NetworkAgentInfo
from mNetworkForNetId so we cannot look up the NetID in there.
There is no harm in allowing notification hiding for networks that
are disconnected as the notification logic does not effect the
Network state (like the validation message that caused the addition
of the Network liveness check).

bug:17261757
Change-Id: Id0a299e230ae37e641ac2faeebc45550e27c1fa4