Clean up wifi/mobile indicators in the expanded panel.

We now have two separate RSSI icons, one for wifi and one
for mobile data, as originally proposed in bug 5544770.
Each has an accompanying label showing the appropriate
network description string (PLMN or SSID or "No internet
connection"); these will be truncated as necessary to avoid
clobbering other UI elements in the panel.

Bug: 5543495
Bug: 5621224
Change-Id: I8fc322f8471fc94000e7230de359d3f1fe561b1e