Retain source data for prepared dictionary.

Unlike pure-java prepared dictionary, in native prepared dictionaries source data is not copied, but referenced.

Likely because of copy-paste, `raw_data` field was not populated.

PiperOrigin-RevId: 944339020

java/org/brotli/wrapper/enc/EncoderJNI.java

@@ -35,6 +35,8 @@ class EncoderJNI {
 
     private PreparedDictionaryImpl(ByteBuffer data, ByteBuffer rawData) {
       this.data = data;
+      // Retain a reference to the raw data to prevent it from being garbage collected.
+      this.rawData = rawData;
     }
 
     @Override