SysUI: Actual MSIM status bar support

Expand SignalClusterView and NetworkController to handle multiple
SIMs.  It does this by creating multiple MobileSignalControllers
for each of the active subscriptions on the device.

Also some minor changes for followup on the NetworkController
refactor that went in before this.

Bug: 18222975
Change-Id: Ic7a857cfc5cadb46d51bb9ded0df8187eea799f7