Account for removing primary subs within a group.

In the DSDS case where there are two subscriptions belonging in the same
group and then one gets removed, we can simply assume that the remaining
sub should become the default without prompting the user.

Test: atest FrameworksTelephonyTests:MultiSimSettingControllerTest
Test: manual
Bug: 142092510
Change-Id: Ibeeec8d42c78059778415f5275bd78f224990135
Merged-In: Ibeeec8d42c78059778415f5275bd78f224990135