[SPARK-14659][ML] RFormula consistent with R when handling strings

## What changes were proposed in this pull request?
When handling strings, the category dropped by RFormula and R are different:
- RFormula drops the least frequent level
- R drops the first level after ascending alphabetical ordering

This PR supports different string ordering types in StringIndexer #17879 so that RFormula can drop the same level as R when handling strings using`stringOrderType = "alphabetDesc"`.

## How was this patch tested?
new tests

Author: Wayne Zhang <actuaryzhang@uber.com>

Closes #17967 from actuaryzhang/RFormula.