Use active instead of available subscriptions in 2 places

In a couple of places we were showing DSDS UI using the list of
*available* subscriptions instead of the list of *active* ones. This
difference is relevant for instance if you have multiple eSIM profiles -
in typical hardware only one of those can be active at one time. This CL
fixes that problem in two places:

1) The header at the top of the Network & internet page

2) The ListPreference's on the mobile network detail page for selecting
   the default subscription for Calls / SMS.

Fixes: 129673046
Test: make RunSettingsRoboTests
Change-Id: I891c856bdd4516286fcee1067684d9161ceaca80