Provide notification handler in device and folder activities

[adamszewe]

wip, waiting for #2054
to merge first

Decouples a bit FolderActivity and DeviceActivity from SyncthingService by injecting NotificationHandler into them directly instead of relying on a method in SyncthingService to provide it for them.

[imsodin]

How does this relate to #2054? Doesn't look like preferences are involved here.

With the previous way, only a single notification handler instance. Is it ok to have multiple?

[adamszewe]

How does this relate to #2054? Doesn't look like preferences are involved here.

With the previous way, only a single notification handler instance. Is it ok to have multiple?

This PR is just waiting for the first one to get merged so I can rebase this one. I left it in draft as it's still pointing to the main branch even though it has changes on it from the first PR.

In this PR I want to refactor how the NotificationHandler is provided the FolderActivity and DeviceActivity.
Currently these classes are getting a reference to the NotificationHandler from SyncthingActivity, which couples them for no good reason.
We can just inject NotificationHandler into those classes.
NotificationHandler is already being provided as a s Singleton by Dagger, so there is going to be only a single instance of it in the app.

[acolomb]

Are you closing this just for lack of review / feedback? Or is there some different plan?