[SPARK-20770][SQL] Improve ColumnStats

## What changes were proposed in this pull request?

This PR improves the implementation of `ColumnStats` by using the following appoaches.

1. Declare subclasses of `ColumnStats` as `final`
2. Remove unnecessary call of `row.isNullAt(ordinal)`
3. Remove the dependency on `GenericInternalRow`

For 1., this declaration encourages method inlining and other optimizations of JIT compiler
For 2., in `gatherStats()`, while previous code in subclasses of `ColumnStats` always calls `row.isNullAt()` twice, the PR just calls `row.isNullAt()` only once.
For 3., `collectedStatistics()` returns `Array[Any]` instead of `GenericInternalRow`. This removes the dependency of unnecessary package and reduces the number of allocations of `GenericInternalRow`.

In addition to that, in the future, `gatherValueStats()`, which is specialized for each data type, can be effectively called from the generated code without using generic data structure `InternalRow`.

## How was this patch tested?

Tested by existing test suite

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

Closes #18002 from kiszk/SPARK-20770.