Change PeriodicSequence to report backlog accurately

[scwhittle]

Instead of reporting all the work it will do as backlog it should report just the scheduled based upon the current time that have not been produced yet.

This is done via GetSize but could be done by changing how Progress is reported. This seems more targeted fix as I'm not sure of all the implications of modifying progress.

Also fixup BOUNDED/UNBOUNDED since an effective end-of-time timestamp can be set.

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[scwhittle]

java sdk changes for #32506

[github-actions]

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[scwhittle]

Verified with manual pipeline runs against Dataflow Runner that downscaling of pipeline with PeriodicSequence works with this change

[Abacn]

thanks! Python SDK has the same implementation, a follow up could be applying the same fix there

[scwhittle]

Run Java_GCP_IO_Direct PreCommit

[scwhittle]

Failure appears unrelated