Changed new Thread() to use new AsyncTask()

Changed new Thread() to use new AsyncTask() - same as was done in src/org/thoughtcrime/securesms/service/KeyCachingService.java - d97252d8d6

src/org/thoughtcrime/securesms/util/Trimmer.java

@@ -25,12 +25,13 @@ public class Trimmer {
     if (!trimmingEnabled)
       return;
 
-    new Thread() {
+    new AsyncTask<Void, Void, Void>() {
       @Override
-      public void run() {
+      protected Void doInBackground(Void... params) {
         DatabaseFactory.getThreadDatabase(context).trimThread(threadId, threadLengthLimit);
+        return null;
       }
-    }.start();
+    }.execute();
   }
 
   private static class TrimmingProgressTask extends AsyncTask<Integer, Integer, Void> implements ThreadDatabase.ProgressListener {