Add an on/off switch to the top of mobile network details page

When a device is in DSDS mode and has two mobile network subscriptions,
a user may want to completely disable one of those subscriptions. This
implements the UI for this, calling into some recently added APIs in the
SubscriptionManager class.

Bug: 122670283
Test: make RunSettingsRoboTests
Change-Id: I40fd3e777ce3542004e4761406b24196505b97d8