Fix Blocky UI behavior while scanning.

onUrlDeviceDiscoveryUpdate being called rapidly during scanning.
Because of inefficiencies in the process of adding it to the UI
this caused blocky behavior. This fix optimizes when we call the
UIThread to empty the GroupIdQueue.

Author: iankchristie

android/PhysicalWeb/app/src/main/java/org/physical_web/physicalweb/NearbyBeaconsFragment.java

@@ -296,32 +296,28 @@ public void onListItemClick(ListView l, View v, int position, long id) {
 
   @Override
   public void onUrlDeviceDiscoveryUpdate() {
+    for (PwPair pwPair : mPwCollection.getGroupedPwPairsSortedByRank(
+        new Utils.PwPairRelevanceComparator())) {
+      String groupId = Utils.getGroupId(pwPair.getPwsResult());
+      Log.d(TAG, "groupid to add " + groupId);
+      if (mNearbyDeviceAdapter.containsGroupId(groupId)) {
+        mNearbyDeviceAdapter.updateItem(pwPair);
+      } else if (!mGroupIdQueue.contains(groupId)
+          && !Utils.isBlocked(pwPair)) {
+        mGroupIdQueue.add(groupId);
+      }
+    }
+
+    if(mGroupIdQueue.isEmpty() || !mSecondScanComplete) {
+      return;
+    }
     // Since this callback is given on a background thread and we want
     // to update the list adapter (which can only be done on the UI thread)
     // we have to interact with the adapter on the UI thread.
     new Handler(Looper.getMainLooper()).post(new Runnable() {
       @Override
       public void run() {
-        if (SwipeDismissListViewTouchListener.isLocked()) {
-          return;
-        }
-        for (PwPair pwPair : mPwCollection.getGroupedPwPairsSortedByRank(
-            new Utils.PwPairRelevanceComparator())) {
-          String groupId = Utils.getGroupId(pwPair.getPwsResult());
-          Log.d(TAG, "groupid to add " + groupId);
-          if (mNearbyDeviceAdapter.containsGroupId(groupId)) {
-            mNearbyDeviceAdapter.updateItem(pwPair);
-          } else if (!mGroupIdQueue.contains(groupId)
-              && !Utils.isBlocked(pwPair)) {
-            mGroupIdQueue.add(groupId);
-            if (mSecondScanComplete) {
-              // If we've already waited for the second scan timeout,
-              // go ahead and put the item in the listview.
-              emptyGroupIdQueue();
-            }
-          }
-        }
-        mNearbyDeviceAdapter.notifyDataSetChanged();
+       emptyGroupIdQueue();
       }
     });
   }