[SPARK-4812][SQL] Fix the initialization issue of 'codegenEnabled'

The problem is `codegenEnabled` is `val`, but it uses a `val` `sqlContext`, which can be override by subclasses. Here is a simple example to show this issue.

```Scala
scala> :paste
// Entering paste mode (ctrl-D to finish)

abstract class Foo {

  protected val sqlContext = "Foo"

  val codegenEnabled: Boolean = {
    println(sqlContext) // it will call subclass's `sqlContext` which has not yet been initialized.
    if (sqlContext != null) {
      true
    } else {
      false
    }
  }
}

class Bar extends Foo {
  override val sqlContext = "Bar"
}

println(new Bar().codegenEnabled)

// Exiting paste mode, now interpreting.

null
false
defined class Foo
defined class Bar
```

We should make `sqlContext` `final` to prevent subclasses from overriding it incorrectly.

Author: zsxwing <zsxwing@gmail.com>

Closes #3660 from zsxwing/SPARK-4812 and squashes the following commits:

1cbb623 [zsxwing] Make `sqlContext` final to prevent subclasses from overriding it incorrectly