title: Make PendingIntent from notification not fire if the Activity is already on front
body: <p><code>Service</code> A creates a <code>Notification</code> with a <code>PendingIntent</code> that opens <code>Activity</code> B. User pulls the notification drawer down and presses the <code>Notification</code> with <code>Activity</code> B on front previously. The <code>PendingIntent</code> basically adds another instance of <code>Activity</code> B instead of opening the currently open one.</p>
  
  <p>Is there any way to make the <code>PendingIntent</code> open the <code>Activity</code> if it's already open, go back on the backstack if there's an instance of that <code>Activity</code> there, and open a new instance otherwise?</p>