MediaBrowserCompat: Supplement JavaDoc of MediaItem.getMediaId()

This CL adds link to MediaMetadataCompat.METADATA_KEY_MEDIA_ID
into the JavaDoc of MediaItem.getMediaId() to help developers
easily find what the MediaId is.

Bug: 28790407
Change-Id: I7ca64e436f8290ed0723daaf19f33e07ad800997
diff --git a/media-compat/java/android/support/v4/media/MediaBrowserCompat.java b/media-compat/java/android/support/v4/media/MediaBrowserCompat.java
index d39a297..b06c8e7 100644
--- a/media-compat/java/android/support/v4/media/MediaBrowserCompat.java
+++ b/media-compat/java/android/support/v4/media/MediaBrowserCompat.java
@@ -457,7 +457,8 @@
         }
 
         /**
-         * Returns the media id for this item.
+         * Returns the media id in the {@link MediaDescriptionCompat} for this item.
+         * @see MediaMetadataCompat#METADATA_KEY_MEDIA_ID
          */
         public @NonNull String getMediaId() {
             return mDescription.getMediaId();