[SPARK-19669][SQL] Open up visibility for sharedState, sessionState, and a few other functions

## What changes were proposed in this pull request?
To ease debugging, most of Spark SQL internals have public level visibility. Two of the most important internal states, sharedState and sessionState, however, are package private. It would make more sense to open these up as well with clear documentation that they are internal.

In addition, users currently have way to set active/default SparkSession, but no way to actually get them back. We should open those up as well.

## How was this patch tested?
N/A - only visibility change.

Author: Reynold Xin <rxin@databricks.com>

Closes #17002 from rxin/SPARK-19669.