Add back old deprecated MediaSource.prepareSource - remove for next release!

This ensures compatiblity of other apps depending on our public GitHub code.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=203129076

library/core/src/main/java/com/google/android/exoplayer2/source/BaseMediaSource.java

@@ -137,6 +137,12 @@ public abstract class BaseMediaSource implements MediaSource {
 
   @Override
   public final void prepareSource(
+      ExoPlayer player, boolean isTopLevelSource, SourceInfoRefreshListener listener) {
+    prepareSource(player, isTopLevelSource, listener, /* mediaTransferListener= */ null);
+  }
+
+  @Override
+  public final void prepareSource(
       ExoPlayer player,
       boolean isTopLevelSource,
       SourceInfoRefreshListener listener,

library/core/src/main/java/com/google/android/exoplayer2/source/MediaSource.java

@@ -190,6 +190,11 @@ public interface MediaSource {
    */
   void removeEventListener(MediaSourceEventListener eventListener);
 
+  /** @deprecated Will be removed in the next release. */
+  @Deprecated
+  void prepareSource(
+      ExoPlayer player, boolean isTopLevelSource, SourceInfoRefreshListener listener);
+
   /**
    * Starts source preparation if not yet started, and adds a listener for timeline and/or manifest
    * updates.