Keep FlacStreamInfo unobfuscated as it is accessed from native methods

Issue: #2427

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146905629

extensions/flac/proguard-rules.txt

@@ -5,7 +5,10 @@
     native <methods>;
 }
 
-# Some members of this class are being accessed from native methods. Keep them unobfuscated.
+# Some members of these classes are being accessed from native methods. Keep them unobfuscated.
 -keep class com.google.android.exoplayer2.ext.flac.FlacDecoderJni {
     *;
 }
+-keep class com.google.android.exoplayer2.util.FlacStreamInfo {
+    *;
+}