Limit VCNs to one running at a given time

This change ensures that there is only ever one VCN running at a given
point in time, and that if the device has switched to using a
subscription in a different subscription group, the VCN will immediately
tear down.

This ensures that when on a DSDS device, when the VCN-enabled
subscription is not the default/active subscription, the other
subscription's network will never be outscored by the VCN, and thus get
torn down.

Bug: 190761448
Test: atest FrameworksVcnTests
Test: Manual testing to ensure common functionality
Original-Change: https://android-review.googlesource.com/1767050
Merged-In: I8031fab7502880d38420058451df41f47567c458
Change-Id: I8031fab7502880d38420058451df41f47567c458