[SPARK-18530][SS][KAFKA] Change Kafka timestamp column type to TimestampType

## What changes were proposed in this pull request?

Changed Kafka timestamp column type to TimestampType.

## How was this patch tested?

`test("Kafka column types")`.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #15969 from zsxwing/SPARK-18530.

(cherry picked from commit d0212eb0)
Signed-off-by: Shixiong Zhu <shixiong@databricks.com>