Sync now: add a toast to give feedback about nothing to sync

SmsSyncAdapter: make android studio happy

Drop .observers.SmsObserverService which is unused

src/main/res/values-en/strings.xml

@@ -171,5 +171,6 @@
     <string name="restore_all_messages">Restore all messages</string>
     <string name="account_actions">Account actions</string>
     <string name="error_connection_failed_not_found">Connection failed, the server returns 404 NOT FOUND. Ensure you set the correct path for your ownCloud instance.</string>
+    <string name="nothing_to_sync">There is nothing to sync.</string>
 
 </resources>

src/main/res/values-fr/strings.xml

@@ -158,4 +158,5 @@
     <string name="restore_all_messages">Restaurer tous les messages</string>
     <string name="account_actions">Actions du compte</string>
     <string name="error_connection_failed_not_found">Echec de connexion, le serveur a retourné 404 NON TROUVE. Assurez vous d\'avoir spécifié la bonne URL.</string>
+    <string name="nothing_to_sync">Il n\'y a rien à synchroniser.</string>
 </resources>

src/main/res/values/strings.xml

@@ -206,4 +206,5 @@
     <string name="restore_all_messages">Restore all messages</string>
     <string name="account_actions">Account actions</string>
     <string name="error_connection_failed_not_found">Connection failed, the server returns 404 NOT FOUND. Ensure you set the correct path for your ownCloud instance.</string>
+    <string name="nothing_to_sync">There is nothing to sync.</string>
 </resources>