Add java docs for ContrastProcessor constructor.

* Add docs for the public constructor for consistency with all the
other frame processors.

PiperOrigin-RevId: 473726586
(cherry picked from commit 7a460fa3)

libraries/effect/src/main/java/androidx/media3/effect/ContrastProcessor.java

@@ -32,6 +32,15 @@ import java.io.IOException;
 
   private final GlProgram glProgram;
 
+  /**
+   * Creates a new instance.
+   *
+   * @param context The {@link Context}.
+   * @param contrastEffect The {@link Contrast} to apply to each frame in order.
+   * @param useHdr Whether input textures come from an HDR source. If {@code true}, colors will be
+   *     in linear RGB BT.2020. If {@code false}, colors will be in gamma RGB BT.709.
+   * @throws FrameProcessingException If a problem occurs while reading shader files.
+   */
   public ContrastProcessor(Context context, Contrast contrastEffect, boolean useHdr)
       throws FrameProcessingException {
     super(useHdr);