Update Notification Body Text

**Is your feature request related to a problem? Please describe.**
Nope, no problems. This is purely a "nice to have" feature.

**Describe the solution you'd like**
I'd like to be able to update the body text of the foreground service notification from within the background watcher callback.

**Describe alternatives you've considered**
The closest alternative I could find is #51, but I don't need anything fancier than just updating the body text that is already there.

**Additional context**
I'm mostly creating this issue to inquire if this is even possible on both Android and iOS, and to see if this feature aligns with this project's scope. If so, I'd love to help get this implemented.