Fix: Use support library Dialog fragment.

On pre marshmallow devices, fragment.onAttach(context) doesn't exist
as it was added in api 23.

Author: Sam1301

app/src/main/java/com/zulip/android/activities/ZulipActivity.java

@@ -6,7 +6,7 @@
 import android.annotation.TargetApi;
 import android.app.AlertDialog;
 import android.app.DatePickerDialog;
-import android.app.DialogFragment;
+import android.support.v4.app.DialogFragment;
 import android.app.NotificationManager;
 import android.app.PendingIntent;
 import android.app.SearchManager;
@@ -570,7 +570,7 @@ public Cursor runQuery(CharSequence charSequence) {
     public void showListDialog() {
         // Create an instance of the dialog fragment and show it
         DialogFragment dialog = new ListDialog();
-        dialog.show(getFragmentManager(), "ListDialogFragment");
+        dialog.show(getSupportFragmentManager(), "ListDialogFragment");
     }
 
     /**

app/src/main/java/com/zulip/android/util/ListDialog.java

@@ -2,7 +2,7 @@
 
 import android.app.AlertDialog;
 import android.app.Dialog;
-import android.app.DialogFragment;
+import android.support.v4.app.DialogFragment;
 import android.content.Context;
 import android.content.pm.PackageManager;
 import android.os.Bundle;