This adds prefs to the the details page for mobile network subscriptions
that appear in multi-SIM mode. They show which subscription is set as
the default for calls and SMS respectively, and let the user change what
the default is for each. They can be set to either a specific
subscription, or set to "Ask every time".
Bug: 122670283
Test: make RunSettingsRoboTests
Change-Id: Id2b23e4b5c3d1b5c04b5766e73fc8202c05d8f75
