validation: only call UpdatedBlockTip for active chainstate

This notification isn't needed for background chainstates.

`kernel::Notifications::blockTip` are also skipped.

src/validationinterface.h

@@ -87,7 +87,7 @@ protected:
      * but may not be called on every intermediate tip. If the latter behavior is desired,
      * subscribe to BlockConnected() instead.
      *
-     * Called on a background thread.
+     * Called on a background thread. Only called for the active chainstate.
      */
     virtual void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {}
     /**