Better detection of AMP environment

[bretg]

Ran across this TODO in the code -- the AMP context is now available according to the AMP issue:

  function isAmp(uuid) {
    // TODO Use amp context once it is available in cross domain
    // https://github.com/ampproject/amphtml/issues/6829
    return typeof uuid === 'string' && uuid !== "" && isCrossDomain();
  }