title: Android - display in the map only the markers included in a determinate area
body: <p>I have my application with a map. 
  In this map i put a marker in the current location of the device. I also add a circle around the marker as follow: </p>
  
  <p><i></p>
  
  <pre><code>    Circle circle = mMap.addCircle(new CircleOptions()
                          .center(latLng)
                          .radius(400)     //The radius of the circle, specified in meters. It should be zero or greater.
                          .strokeColor(Color.rgb(0, 136, 255))
                          .fillColor(Color.argb(20, 0, 136, 255)));
  </code></pre>
  
  <p></i>
  The result is something like this:
  <br>
  <a href="http://i.stack.imgur.com/K7s0p.png" rel="noreferrer" title="check it">here's an example of the result</a>!</p>
  
  <p>I have a database with some positions characterized by a latitude and a longitude. </p>
  
  <p>I would set markers in the map, only for positions that are located within the circle added previously. 
  <br><strong>How can I understand which of them are included in that area?</strong></p>
  
  <p>please help me, thanks!</p>