Create private String KEY about activitystarter

[MarcinMoskala]

Static field with the key is a good idea, but I am skipping prefix because it is easy to replace with custom key and this is complicating both library and it's usage.

[MarcinMoskala]

I am not sure if a field with the key should be public. Why? Are there any real use case when it is needed?

[Kolyall]

You can make it optional... for example access=PUBLIC

[MarcinMoskala]

It won't change anything. The only reason I see that this keys could be useful is when the user needs to change how properties storage is working. And I think that it should never happen. It is because flow might change inside the library and when the developer changed it (for any reason) and then flow is changing then the user has a problem. Possibly he is even not the same developer then one who made a workaround in the first place. If there are any use-cases that need flow to be different then we should work on supporting them.