[SPARK-11681][STREAMING] Correctly update state timestamp even when state is not updated

Bug: Timestamp is not updated if there is data but the corresponding state is not updated. This is wrong, and timeout is defined as "no data for a while", not "not state update for a while".

Fix: Update timestamp when timestamp when timeout is specified, otherwise no need.
Also refactored the code for better testability and added unit tests.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #9648 from tdas/SPARK-11681.