Avoid duplicate idle timers in receiver app.

It is possible to call startIdleTimer_ multiple times in a row.  This
change makes sure that there is only one idle timer at a time.
Otherwise there is no way to cancel the other timers.

Closes #684

Change-Id: I264c7e2edadea76978c4f3848f27bb13c29510dd

demo/receiver_app.js

@@ -132,6 +132,8 @@ ShakaReceiver.prototype.checkIdle_ = function() {
 
 /** @private */
 ShakaReceiver.prototype.startIdleTimer_ = function() {
+  this.cancelIdleTimer_();
+
   this.idleTimerId_ = window.setTimeout(
       window.close.bind(window), this.idleTimeout_ * 1000.0);
 };