Send broadcasts when VPNs come and go.

This is achieved by adding TYPE_VPN as a supported legacy type.
Note that this is added in code, and not in a config.xml file,
so there's no way to remove TYPE_VPN (i.e., make it unsupported).

Bug: 17426546
Change-Id: I02eb5f22737720095f646f8db5c87fd66da129d6