[SPARK-15960][SQL] Rename `spark.sql.enableFallBackToHdfsForStats` config

## What changes were proposed in this pull request?
Since we are probably going to add more statistics related configurations in the future, I'd like to rename the newly added `spark.sql.enableFallBackToHdfsForStats` configuration option to `spark.sql.statistics.fallBackToHdfs`. This allows us to put all statistics related configurations in the same namespace.

## How was this patch tested?
None - just a usability thing

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #13681 from hvanhovell/SPARK-15960.