feat: add isScheduled call

[mx1up]

checks whether a period task is scheduled. Otherwise, there is no way to know whether some work you previously might have scheduled, is actually scheduled or not.

Since periodic tasks are android only, this call is also android only.

fixes #534

related #475

[zzzh]

Hello, @mx1up ,

I need this too, when will it be ready to release?

[mx1up]

to me it's ready, just waiting for one of the maintainers to chime in ;)

[zzzh]

to me it's ready, just waiting for one of the maintainers to chime in ;)

Cool, looks like someone was sleeping, for weeks!

[Prince2347X]

Hey @mx1up, thank you for adding the isScheduledByUniqueName() method. Can we get the state of the task, instead of just a boolean telling if the work is scheduled or not, by implementing the native getWorkInfosForUniqueWork() to flutter. That would be a great addition. Please look into it if you have time.

[ened]

@mx1up could you please rebase & update the inline docs so it's clear this is a Android-only method?