use synchronized for methods that override synchronized methods

app/src/main/java/org/fdroid/fdroid/ProgressBufferedInputStream.java

@@ -24,7 +24,7 @@ class ProgressBufferedInputStream extends BufferedInputStream {
     }
 
     @Override
-    public int read(@NonNull byte[] buffer, int byteOffset, int byteCount) throws IOException {
+    public synchronized int read(@NonNull byte[] buffer, int byteOffset, int byteCount) throws IOException {
         if (progressListener != null) {
             currentBytes += byteCount;
             /* don't send every change to keep things efficient.  333333 bytes to keep all

app/src/main/java/org/fdroid/fdroid/net/Downloader.java

@@ -256,7 +256,7 @@ public abstract class Downloader {
         }
 
         @Override
-        public void mark(int readlimit) {
+        public synchronized void mark(int readlimit) {
             toWrap.mark(readlimit);
         }