2. The reason is for simplicity of interface. 
The goal of my change is to provide a simple way to the customer to set the flush (AbstractTdLogger's worker) and upload (TdLoggerService worker) frequency, whether for debugging reasons, or performance tuning, or others.
To be able to set the worker interval in the worker that is instantiated in the TdLoggerService Service, I will need to provide a "user space" API, namely in AbstractTdLogger/DefaultTdLogger, since I believe an application has no direct access to the Service's TdLoggerService instance object. 
To me that seemed a big violation of separation of duties where code in the Logger would modify "things" in the Service.

I opted to use a static method and static variable so that once the interval is set once, it would apply to all the RepeatingWorker's instances "immediately" (not really immediately, at the next interval).

I'm sure you can may have a better idea and if you do, please let me know.