update proguard to fix R8 compatibility issue (#1527)

https://r8.googlesource.com/r8/+/refs/heads/master/compatibility-faq.md#member-in-a-data-object-is-always

examples/android-proguard-example/proguard.cfg

@@ -19,4 +19,9 @@
 -keep class * implements com.google.gson.JsonSerializer
 -keep class * implements com.google.gson.JsonDeserializer
 
+# Prevent R8 from leaving Data object members always null
+-keepclassmembers,allowobfuscation class * {
+  @com.google.gson.annotations.SerializedName <fields>;
+}
+
 ##---------------End: proguard configuration for Gson  ----------