Fix mobile data on/off switch for multi-sim case

In multi-sim, the on/off switch for mobile data on the mobile network
details page doesn't quite work right. The thing it's missing is
checking whether the current sim is the default one used for data - if
it's not the default, then the switch should be off even if mobile data
is turned on. And clicking to enable needs to prompt the user to ask if
this sim should be become the new default one, which we already have
code for, but we weren't triggering it properly.

Bug: 124005813
Test: make RunSettingsRoboTests
Change-Id: Ifdf58addfde94b3bac85c68e4b22ddfbaf01dff7