[SPARK-18118][SQL] fix a compilation error due to nested JavaBeans

## What changes were proposed in this pull request?

This PR avoids a compilation error due to more than 64KB Java byte code size. This error occur since generated java code `SpecificSafeProjection.apply()` for nested JavaBeans is too big. This PR avoids this compilation error by splitting a big code chunk into multiple methods by calling `CodegenContext.splitExpression` at `InitializeJavaBean.doGenCode`
An object reference for JavaBean is stored to an instance variable `javaBean...`. Then, the instance variable will be referenced in the split methods.

Generated code with this PR
````
/* 22098 */   private void apply130_0(InternalRow i) {
...
/* 22125 */     boolean isNull238 = i.isNullAt(2);
/* 22126 */     InternalRow value238 = isNull238 ? null : (i.getStruct(2, 3));
/* 22127 */     boolean isNull236 = false;
/* 22128 */     test.org.apache.spark.sql.JavaDatasetSuite$Nesting1 value236 = null;
/* 22129 */     if (!false && isNull238) {
/* 22130 */
/* 22131 */       final test.org.apache.spark.sql.JavaDatasetSuite$Nesting1 value239 = null;
/* 22132 */       isNull236 = true;
/* 22133 */       value236 = value239;
/* 22134 */     } else {
/* 22135 */
/* 22136 */       final test.org.apache.spark.sql.JavaDatasetSuite$Nesting1 value241 = false ? null : new test.org.apache.spark.sql.JavaDatasetSuite$Nesting1();
/* 22137 */       this.javaBean14 = value241;
/* 22138 */       if (!false) {
/* 22139 */         apply25_0(i);
/* 22140 */         apply25_1(i);
/* 22141 */         apply25_2(i);
/* 22142 */       }
/* 22143 */       isNull236 = false;
/* 22144 */       value236 = value241;
/* 22145 */     }
/* 22146 */     this.javaBean.setField2(value236);
/* 22147 */
/* 22148 */   }
...
/* 22928 */   public java.lang.Object apply(java.lang.Object _i) {
/* 22929 */     InternalRow i = (InternalRow) _i;
/* 22930 */
/* 22931 */     final test.org.apache.spark.sql.JavaDatasetSuite$NestedComplicatedJavaBean value1 = false ? null : new test.org.apache.spark.sql.JavaDatasetSuite$NestedComplicatedJavaBean();
/* 22932 */     this.javaBean = value1;
/* 22933 */     if (!false) {
/* 22934 */       apply130_0(i);
/* 22935 */       apply130_1(i);
/* 22936 */       apply130_2(i);
/* 22937 */       apply130_3(i);
/* 22938 */       apply130_4(i);
/* 22939 */     }
/* 22940 */     if (false) {
/* 22941 */       mutableRow.setNullAt(0);
/* 22942 */     } else {
/* 22943 */
/* 22944 */       mutableRow.update(0, value1);
/* 22945 */     }
/* 22946 */
/* 22947 */     return mutableRow;
/* 22948 */   }
````

## How was this patch tested?

added a test suite into `JavaDatasetSuite.java`

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #16032 from kiszk/SPARK-18118.