Avoid memcpy in JavaToNativeEncodedImage

Followup to https://webrtc-review.googlesource.com/c/src/+/142160

Bug: webrtc:9378
Change-Id: If790cd628433046d6819a92449fcc68106535df4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/154561
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29359}

sdk/android/api/org/webrtc/EncodedImage.java

@@ -71,6 +71,7 @@ public class EncodedImage implements RefCounted {
 
   // A false return value means that the encoder expects that the buffer is no longer used after
   // VideoEncoder.Callback.onEncodedFrame returns.
+  @CalledByNative
   boolean maybeRetain() {
     if (supportsRetain) {
       retain();