Add proguard config for VP9 extension

extensions/vp9/src/main/proguard.cfg

+# Proguard rules specific to the VP9 extension.
+
+# This prevents the names of native methods from being obfuscated.
+-keepclasseswithmembernames class * {
+    native <methods>;
+}
+
+# Some members of this class are being accessed from native methods. Keep them unobfuscated.
+-keep class com.google.android.exoplayer.ext.vp9.VpxDecoderWrapper$OutputBuffer {
+    *;
+}