Fix / Serialization issues

Fixes  keybackup import failing and UTD of incoming messages

matrix-sdk-android/proguard-rules.pro

@@ -64,3 +64,19 @@
 
 ### Webrtc
 -keep class org.webrtc.** { *; }
+
+### Serializable persisted classes
+# https://www.guardsquare.com/en/products/proguard/manual/examples#serializable
+-keepnames class * implements java.io.Serializable
+
+-keepclassmembers class * implements java.io.Serializable {
+    static final long serialVersionUID;
+    private static final java.io.ObjectStreamField[] serialPersistentFields;
+    !static !transient <fields>;
+    !private <fields>;
+    !private <methods>;
+    private void writeObject(java.io.ObjectOutputStream);
+    private void readObject(java.io.ObjectInputStream);
+    java.lang.Object writeReplace();
+    java.lang.Object readResolve();
+}