enhance logging on android part of plugin to be production-ready

[tiholic]

Convert all System.out.println to a production ready logging system, probably SLF4J as suggested by QuintinWillison in this thread: https://github.com/ably/ably-flutter/pull/21#discussion_r462134457

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[ikbalkaya]

Searched the whole plugin for "System.out.*" and only one result appeared System.out.printf("%splatform method not implemented. %n", methodName); In other places it's standard Android logging Log.x. So I will assume that this is still relevant and we want to adopt a standardised logging system we do (maybe we already did) in Kotlin/Java libraries

[QuintinWillison]

Relates to https://github.com/ably/ably-java/issues/64 - though, really, the crux of that is whether we should be adopting SLF4J (LOG4J is just an implementation... vulnerable to flaws, like any other implementation).