[FLINK-38932][runtime] Fix incorrect scheduled timestamp in ProcessingTimeCallback with scheduleWithFixedDelay

[X-czh]

What is the purpose of the change

Fix incorrect scheduled timestamp in ProcessingTimeCallback with scheduleWithFixedDelay.

Verifying this change

This change is a trivial rework.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[X-czh]

@Zakelly @xiangforever2014 Would you mind taking a look?

[flinkbot]

CI report:

• 7906400 Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[davidradl]

Please could you add a test for this change

[X-czh]

Sure

[davidradl]

I am checking the logic on the first time - I would expect the initial delay to take effect, then subsequent time the fixed delay. As the code is written the first time we get the initial plus the fixed delay, can you confirm this is what you want?

[X-czh]

The initial delay is accounted when calling scheduleRepeatedly, where nextTimestamp is initialized to getCurrentProcessingTime() + initialDelay