Resolve the endpoint in legacy VPN

This adds code to resolve the endpoint in the legacy VPN
runner if it was specified as a hostname, and enables the
previously added test that was disabled because this was
broken until this patch. See the linked bug for details.

This patch uses the async DNS API to do the resolution.
This lets the resolution be fully cancellable, though the
code is more complex than with the non-interruptible
getByName.

Test: VpnTest and in particular VpnTest#testStartRacoon
      Fixes the test meant to test this
      Also manual testing that resolution of a real
      hostname works as expected, that failure to resolve
      returns correctly, and that cancellation/interruption
      will unblock the thread and terminate immediately.
Bug: 158974172
Change-Id: I714985f3c7919dad9c1854830c50f29c1f94a21e