Add a plus button to the mobile pref on Network & internet page

On the Network & internet page, we have a "Mobile network" pref that in
single-SIM mode leads to a detail page for the current SIM. In multi-SIM
mode it has more complicated behavior (leading either to details about
the current subscription if there is only one, or a list of
subscriptions if more than one).

One of the things we wanted to add was a shortcut to add another eSIM
subscription. So this CL adds a plus button control on the right of the
preference which leads to a flow to add another mobile subscription via
the eSIM manager.

Bug: 116349402
Test: make RunSettingsRoboTests
Change-Id: I38e0031e3bd603e93c45dcb4557750e7bc1b8b5a