Bump "Error Prone" to 2.0.15

and fix a few things :)

smack-im/src/main/java/org/jivesoftware/smack/chat/ChatManager.java

@@ -151,6 +151,7 @@ public final class ChatManager extends Manager{
         // Add a listener for all message packets so that we can deliver
         // messages to the best Chat instance available.
         connection.addSyncStanzaListener(new StanzaListener() {
+            @Override
             public void processStanza(Stanza packet) {
                 Message message = (Message) packet;
                 Chat chat;

smack-im/src/main/java/org/jivesoftware/smack/roster/RosterEntry.java

@@ -222,6 +222,7 @@ public final class RosterEntry extends Manager {
         connection().sendStanza(unsubscribed);
     }
 
+    @Override
     public String toString() {
         StringBuilder buf = new StringBuilder();
         if (getName() != null) {
@@ -249,6 +250,7 @@ public final class RosterEntry extends Manager {
         return getJid().hashCode();
     }
 
+    @Override
     public boolean equals(Object object) {
         if (this == object) {
             return true;

smack-im/src/main/java/org/jivesoftware/smack/roster/packet/RosterPacket.java

@@ -270,6 +270,7 @@ public class RosterPacket extends IQ {
             groupNames.remove(groupName);
         }
 
+        @Override
         public XmlStringBuilder toXML() {
             XmlStringBuilder xml = new XmlStringBuilder(this);
             xml.attribute("jid", jid);

smack-im/src/main/java/org/jivesoftware/smack/roster/rosterstore/DirectoryRosterStore.java

@@ -185,9 +185,11 @@ public final class DirectoryRosterStore implements RosterStore {
         resetEntries(Collections.<Item>emptyList(), "");
     }
 
+    @SuppressWarnings("DefaultCharset")
     private static Item readEntry(File file) {
         Reader reader;
         try {
+            // TODO: Should use Files.newBufferedReader() but it is not available on Android.
             reader = new FileReader(file);
         } catch (FileNotFoundException e) {
             LOGGER.log(Level.FINE, "Roster entry file not found", e);