video: drain libavcodec's reorder buffer at end-of-stream

Closes H1 from the pre-ship review (the known-limit doc note added in
c0d55ba). The previous workaround was "first play-through truncates
the last ~16 frames; replay is fine because flush_buffers clears
libavcodec." That trade-off was OK for shipping but the proper fix
is to drain the reorder buffer before propagating EOS to ExoPlayer.

Media3's SimpleDecoder short-circuits the end-of-stream input buffer
and never invokes the subclass's decode(), so there's no hook to send
avcodec_send_packet(NULL). Every override worth overriding (decode
loop, queue methods, flush) is final on SimpleDecoder. So we
vendor a copy as FfmpegSimpleDecoder (Apache 2.0 attribution at the
top of the file) with one structural change: an EOS-drain state. On
EOS input, signalEndOfInput() flushes libavcodec's reorder queue,
then drainAtEndOfStream() is called on successive output buffers
until it reports DRAIN_DONE — at which point the loop attaches
BUFFER_FLAG_END_OF_STREAM and resumes normal teardown.

Everything else mirrors SimpleDecoder verbatim so upstream
improvements are cheap to pull forward.

  - FfmpegSimpleDecoder.java: vendored base class.
  - ffmpegVideoSignalEos JNI: sends avcodec_send_packet(NULL).
  - FfmpegVideoDecoder: extends the new base; signalEndOfInput
    forwards to the JNI; drainAtEndOfStream re-uses the existing
    ffmpegVideoReceiveFrame so per-frame PTS recovery and the
    pending_frame path from c0d55ba continue to work during drain.

android/ffmpeg/ffmpeg_jni.cc

@@ -444,6 +444,26 @@ VIDEO_DECODER_FUNC(jint, ffmpegVideoReceiveFrame, jlong handle,
   return VIDEO_DECODER_SUCCESS;
 }
 
+// Signals libavcodec to flush its reorder buffer so the Java side can
+// pull the remaining frames out via successive ffmpegVideoReceiveFrame
+// calls. Used during end-of-stream drain: SimpleDecoder's EOS short-
+// circuit hides the EOS input from our decode() override, so without
+// this hook libavcodec's buffered tail (~16 frames for iOS H.264
+// High@3.1) would be lost on first play-through.
+VIDEO_DECODER_FUNC(jint, ffmpegVideoSignalEos, jlong handle) {
+  if (!handle) {
+    LOGE("ffmpegVideoSignalEos: null handle");
+    return VIDEO_DECODER_ERROR_OTHER;
+  }
+  UxFfmpegVideoContext* ctx = (UxFfmpegVideoContext*)handle;
+  int result = avcodec_send_packet(ctx->codec_ctx, nullptr);
+  if (result < 0 && result != AVERROR_EOF) {
+    logError("avcodec_send_packet(NULL)", result);
+    return transformError(result);
+  }
+  return VIDEO_DECODER_SUCCESS;
+}
+
 VIDEO_DECODER_FUNC(void, ffmpegVideoFlush, jlong handle) {
   if (!handle) return;
   UxFfmpegVideoContext* ctx = (UxFfmpegVideoContext*)handle;