Re-add setMediaCas javadoc

Was removed as part of deprecation and is now re-added as a result of API council feedback.

Bug: 210622516
Change-Id: I9076072058c7dd5f98db4b99793f717eea4cf694
Test: N/A.

media/java/android/media/MediaExtractor.java

@@ -250,6 +250,13 @@ public final class MediaExtractor {
             @NonNull FileDescriptor fd, long offset, long length) throws IOException;
 
     /**
+     * Sets the MediaCas instance to use. This should be called after a successful setDataSource()
+     * if at least one track reports mime type of
+     * {@link android.media.MediaFormat#MIMETYPE_AUDIO_SCRAMBLED} or
+     * {@link android.media.MediaFormat#MIMETYPE_VIDEO_SCRAMBLED}. Stream parsing will not proceed
+     * until a valid MediaCas object is provided.
+     *
+     * @param mediaCas the MediaCas object to use.
      * @deprecated Use the {@code Descrambler} system API instead, or DRM public APIs like
      *             {@link MediaDrm}.
      */