sync: dropbox: Cleanup nullable analyzer warnings

sync/src/main/java/com/jefftharris/passwdsafe/sync/dropbox/DropboxCoreProvider.java

@@ -496,8 +496,9 @@ protected TokenRevokeTask(DropboxCoreProvider provider)
             itsProvider = new ManagedRef<>(provider);
         }
 
+        @Nullable
         @Override
-        protected Void doInBackground(DbxClientV2... clients)
+        protected Void doInBackground(@NonNull DbxClientV2... clients)
         {
             PasswdSafeUtil.dbginfo(TAG, "revoking auth tokens");
             for (DbxClientV2 client: clients) {

sync/src/main/java/com/jefftharris/passwdsafe/sync/dropbox/DropboxCoreSyncer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (©) 2017 Jeff Harris <[email protected]>
+ * Copyright (©) 2017-2024 Jeff Harris <[email protected]>
  * All rights reserved. Use of the code is allowed under the
  * Artistic License 2.0 terms, as specified in the LICENSE file
  * distributed with this code, or available from
@@ -9,6 +9,9 @@
 
 import android.content.Context;
 
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
 import com.dropbox.core.DbxException;
 import com.dropbox.core.v2.DbxClientV2;
 import com.dropbox.core.v2.files.FileMetadata;
@@ -47,7 +50,8 @@ public DropboxCoreSyncer(DbxClientV2 client,
     }
 
     /** Create a remote identifier from the local name of a file */
-    public static String createRemoteIdFromLocal(DbFile dbfile)
+    @NonNull
+    public static String createRemoteIdFromLocal(@NonNull DbFile dbfile)
     {
         return ProviderRemoteFile.PATH_SEPARATOR +
                dbfile.itsLocalTitle.toLowerCase();
@@ -56,15 +60,17 @@ public static String createRemoteIdFromLocal(DbFile dbfile)
     /**
      * Get the account display name
      */
-    public static String getDisplayName(DbxClientV2 client) throws DbxException
+    @NonNull
+    public static String getDisplayName(@NonNull DbxClientV2 client)
+            throws DbxException
     {
         FullAccount acct = client.users().getCurrentAccount();
         return acct.getName().getDisplayName();
     }
 
 
     @Override
-    protected SyncRemoteFiles getSyncRemoteFiles(List<DbFile> dbfiles)
+    protected SyncRemoteFiles getSyncRemoteFiles(@NonNull List<DbFile> dbfiles)
             throws DbxException
     {
         SyncRemoteFiles files = new SyncRemoteFiles();
@@ -134,6 +140,7 @@ protected SyncRemoteFiles getSyncRemoteFiles(List<DbFile> dbfiles)
      * @return The file's FileMetadata if found; null or DeletedMetadata if
      * not found
      */
+    @Nullable
     private Metadata getRemoteFile(String remoteId)
             throws DbxException
     {