Check permissions in INetworkManagementService#setIPv6AddrGenMode

This function was missing a permission check, and thus would
allow any app to transitively call the netd setIPv6AddrGenMode
IPC.

Fortunately, due to a bug in that IPC, it can only set the mode
to stable privacy and not to EUI-64.

This code is unused. The only thing on the system that sets the
IPv6 addrgen mode is the networkstack, but that does so by
calling the netd IPC directly.

Test: builds
Bug: 141920289
Change-Id: Id54431e81dceaff09f785a280ceee0973543a30f