Export r8 rules to users

Otherwise r8 will always strip the generated code.

Test: m
Change-Id: I28b23f44961a718461d1842a8682e78078cc6658
diff --git a/Android.bp b/Android.bp
index d6c6f8d..51b8635 100644
--- a/Android.bp
+++ b/Android.bp
@@ -13,6 +13,10 @@
         "-opt-in=kotlinx.serialization.ExperimentalSerializationApi",
         "-opt-in=kotlinx.serialization.InternalSerializationApi",
     ],
+    optimize: {
+        proguard_flags_files: ["rules/*"],
+        export_proguard_flags_files: true,
+    },
     apex_available: [
         "//apex_available:platform",
         "//apex_available:anyapex",