[Media>UI] Don't show media metadata on lockscreen in Incognito mode

Media notification content is marked as private in Incognito mode, so the
content is hidden on lockscreen. For the same reason, we shouldn't show the
artwork image on lockscreen and wearable devices.

BUG=683111

Review-Url: https://codereview.chromium.org/2646793006
Cr-Commit-Position: refs/heads/master@{#445745}
(cherry picked from commit 233f53a)

Review-Url: https://codereview.chromium.org/2657653002 .
Cr-Commit-Position: refs/branch-heads/2987@{#65}
Cr-Branched-From: ad51088-refs/heads/master@{#444943}

chrome/android/java/src/org/chromium/chrome/browser/media/ui/MediaNotificationManager.java

@@ -673,7 +673,10 @@ private void hideNotification(int tabId) {
         clearNotification();
     }
 
+    @Nullable
     private MediaMetadataCompat createMetadata() {
+        if (mMediaNotificationInfo.isPrivate) return null;
+
         MediaMetadataCompat.Builder metadataBuilder = new MediaMetadataCompat.Builder();
 
         metadataBuilder.putString(MediaMetadataCompat.METADATA_KEY_TITLE,