Add to VpnManager the VPN APIs used by Settings and SystemUI.

Settings and SystemUI use these through raw AIDL calls.
Make them proper manager calls so we can move the implementation
without touching the callers again.

For now these still call into ConnectivityService via
IConnectivityManager. In an upcoming CL the implementation will
move to VpnManagerService.

Test: m
Bug: 173331190
Change-Id: I91528e1ad9948fbaa1fc5e37c61c5eb95f54964e