[Java][Bazel] Refine auto-generated pom files (#4780)

java/runtime/pom_template.xml

@@ -1,4 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
+{auto_gen_header}
 <project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns="http://maven.apache.org/POM/4.0.0"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
@@ -25,7 +26,7 @@
       <groupId>org.apache.arrow</groupId>
       <artifactId>arrow-plasma</artifactId>
     </dependency>
-    {generated_bzl_deps}
+{generated_bzl_deps}
   </dependencies>
 
   <build>