[AC-1354] View center property does not work in Mobile Web

<Alloy>
  <Window class="container" layout="absolute">
    <View id="red" backgroundColor="red"></View>
    <View id="green" backgroundColor="green"></View>
    <View id="blue" backgroundColor="blue"></View>
    <View id="yellow" backgroundColor="yellow"></View>
  </Window>
</Alloy>

".container": {
  backgroundColor:"white"
}

"View": {
  height: 150,
  width: 150,
  opacity: 0.7
}

"#red": {
  top: 150,
  left: 150
}

"#green": {
  bottom: 150,
  right: 150
}

"#blue": {
  height: 100,
  width: 100,

  center: {
    x: 150,
    y: 150
  }
}