[SPARK-21046][SQL] simplify the array offset and length in ColumnVector

## What changes were proposed in this pull request?

Currently when a `ColumnVector` stores array type elements, we will use 2 arrays for lengths and offsets and implement them individually in on-heap and off-heap column vector.

In this PR, we use one array to represent both offsets and lengths, so that we can treat it as `ColumnVector` and all the logic can go to the base class `ColumnVector`

## How was this patch tested?

existing tests.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #18260 from cloud-fan/put.