Downloader: Fix crash on loading unfinished downloads from .giga files

This is a fixup, which fixes the crash in release builds, too. It keeps proguard from removing the new method "private void readObject(ObjectInputStream)", which is only called by the VM, but not from the code.

app/proguard-rules.pro

@@ -42,3 +42,9 @@
 -dontwarn javax.annotation.**
 # A resource is loaded with a relative path so the package of this class must be preserved.
 -keepnames class okhttp3.internal.publicsuffix.PublicSuffixDatabase
+-keepclassmembers class * implements java.io.Serializable {
+    static final long serialVersionUID;
+    !static !transient <fields>;
+    private void writeObject(java.io.ObjectOutputStream);
+    private void readObject(java.io.ObjectInputStream);
+}