[Java][FlexBuffers] Make FlexBuffersBuilder reusable by adding clear() (#5889) (#5890)

java/com/google/flatbuffers/ArrayReadWriteBuf.java

@@ -34,6 +34,11 @@ public class ArrayReadWriteBuf implements ReadWriteBuf {
     this.writePos = startPos;
   }
 
+  @Override
+  public void clear() {
+    this.writePos = 0;
+  }
+
   @Override
   public boolean getBoolean(int index) {
     return buffer[index] != 0;