Appsflyer: added httpFallback functionality

@f2prateek the code equivalent to [metadata](https://github.com/segmentio/integration-metadata/pull/232) and [docs](https://github.com/segmentio/site-docs/pull/337)

Am I correct to use getBoolean here?

@harrietgrace 
Needs a default value, should be false in this case.
`settings.getBoolean("httpFallback", false);`