Throw UnsupportedOperationException if startLegacyVpn is called

Legacy VPN is deprecated starting from Android S. So this API
shouldn't be called if the initial SDK version of device is
Android S+. Otherwise, UnsupportedOperationException will be
thrown.

Bug: 189808606
Test: 1. make gts && gts-tradefed
      2. run gts --module GtsNetworkStackHostTestCases
Change-Id: I00f49555bbbc8ccf270056f0e4d1bfdd927ef26d