Run all AsyncTasks on thread pool executor

Should Fix #7002

src/org/thoughtcrime/securesms/giph/net/GiphyProxySelector.java

@@ -36,7 +36,7 @@ public class GiphyProxySelector extends ProxySelector {
           }
           return null;
         }
-      }.execute();
+      }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     } else {
       initializeGiphyProxy();
     }

src/org/thoughtcrime/securesms/giph/ui/GiphyActivity.java

@@ -126,7 +126,7 @@ public class GiphyActivity extends PassphraseRequiredActionBarActivity
           Log.w(TAG, "Resolved Uri is no longer the selected element...");
         }
       }
-    }.execute();
+    }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
   }
 
   private static class GiphyFragmentPagerAdapter extends FragmentPagerAdapter {

src/org/thoughtcrime/securesms/giph/ui/GiphyFragment.java

@@ -117,7 +117,7 @@ public abstract class GiphyFragment extends Fragment implements LoaderManager.Lo
         protected void onPostExecute(List<GiphyImage> images) {
           giphyAdapter.addImages(images);
         }
-      }.execute();
+      }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
   }
 }