SystemUI: Provide correct resource type

- The value stored for when the network
class is unknown is 0. We attempt to get
the resource for this value, which doesn't exist,
resulting in a crash
- Store an empty string for when the
network class is unknown, which is also the
default value.

Change-Id: I1c10dfe18b7dd78e798c2b5a89ca2974136b7e84
CRs-Fixed: 953821