Fixed keeping INSTANCE field and serializer function for serializable objects in R8 full mode (#2865)

Details https://issuetracker.google.com/issues/379996140

Fixes #2861

Author: shanshin

rules/r8.pro

@@ -10,3 +10,16 @@
 
  -if @kotlinx.serialization.Serializable class **
  -keep, allowshrinking, allowoptimization, allowobfuscation, allowaccessmodification class <1>
+
+
+# Rule to save INSTANCE field and serializer function for Kotlin serializable objects.
+#
+# R8 full mode works differently if the instance is not explicitly accessed in the code.
+#
+# see https://github.com/Kotlin/kotlinx.serialization/issues/2861
+# see https://issuetracker.google.com/issues/379996140
+
+-keepclassmembers @kotlinx.serialization.Serializable class ** {
+    public static ** INSTANCE;
+    kotlinx.serialization.KSerializer serializer(...);
+}