Split VideoSamplePipeline from TransformerTranscodingVideoRenderer.

The `VideoSamplePipeline` handles all steps from decoding to
re-encoding that where previously in
`TransformerTranscodingVideoRenderer`. The renderer is now only
responsible for reading the format, reading input, passing it to the
pipeline and passing the pipeline's output to the muxer.

When no transformations are needed, decoding and re-encoding is
skipped using the `PassthroughPipeline`.

PiperOrigin-RevId: 408619407

libraries/transformer/src/main/java/androidx/media3/transformer/AudioSamplePipeline.java

@@ -18,6 +18,7 @@ package androidx.media3.transformer;
 
 import static androidx.media3.common.util.Assertions.checkNotNull;
 import static androidx.media3.common.util.Assertions.checkState;
+import static androidx.media3.common.util.Assertions.checkStateNotNull;
 import static java.lang.Math.min;
 
 import android.media.MediaCodec.BufferInfo;
@@ -157,9 +158,7 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
 
   @Override
   public void releaseOutputBuffer() {
-    if (encoder != null) {
-      encoder.releaseOutputBuffer();
-    }
+    checkStateNotNull(encoder).releaseOutputBuffer();
   }
 
   /**