Add page listing multiple mobile networks

When a device supports simultaneous connection to multiple mobile
networks, we want the "Mobile network" entry on the Network & internet
page to list the number of active SIMs and when clicked, go to a page
that lists them all.

Bug: 116349402
Test: make RunSettingsRoboTests
Change-Id: Ie642d7801cda07dcbbe74d42c234db6605566be4