Add proguard config for Opus extension.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112776882

extensions/opus/src/main/proguard.cfg

+# Proguard rules specific to the Opus extension.
+
+# This prevents the names of native methods from being obfuscated.
+-keepclasseswithmembernames class * {
+    native <methods>;
+}