Make taps on subscriptions in multi-network header go to mobile page

This CL does 2 things:

-Makes the MobileNetworkActivity class capable of showing details for
 "available" in addition to just "active" networks. This is useful for
 dual-standby devices where one subscription is active and another is
 registered but not currently active. It also changes the title of this
 screen to be the network display name (defaults to the carrier name
 but can be customized by the user) instead of the generic "Mobile
 network".

-Wires up the subscription entries in the multi-network header (which
 only appears when a device has multiple subscriptions) so that taps on
 a subscription preference bring you to the MobileNetworkActivity screen
 to show details for that network.

Bug: 116349402
Test: make RunSettingsRoboTests
Change-Id: I0e985652c1d8ec3c597b6b6e4426d222e2ad5352