Suppress warnings in ImaUtil

ImaUtil calls VideoProgressUpdate.equals() which is annotated as hidden,
which causes lint errors with gradle.

#minor-release

PiperOrigin-RevId: 504306210
(cherry picked from commit f86948f01c)

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaUtil.java

@@ -270,6 +270,7 @@ import java.util.Set;
   }
 
   /** Returns a human-readable representation of a video progress update. */
+  @SuppressWarnings("RestrictedApi") // VideoProgressUpdate.equals() is annotated as hidden.
   public static String getStringForVideoProgressUpdate(VideoProgressUpdate videoProgressUpdate) {
     if (VideoProgressUpdate.VIDEO_TIME_NOT_READY.equals(videoProgressUpdate)) {
       return "not ready";