Remove engine dependency of LifecycleChannel

[swift-kim]

Sends lifecycle messages through a BasicMessageChannel instead of using FlutterTizenEngine::SendPlatformMessage.

I implemented StringCodec from scratch because there was no equivalent of the framework's StringCodec in cpp_client_wrapper. StandardCodecSerializer already has support for the string data type but its serialization format is slightly different from StringCodec's.

@flutter-tizen/maintainers How do you feel about this change? I can discard this change if it doesn't make very much sense.

[bwikbs]

First of all, Thank you for your efforts to improve code. 👏 (I should do too... but it's not easy... 😄 )

In my case, I am not sure which one is better, so I referred to other implementation...but I still don't know. 😢
I'm really curious what other people think regarding this.

[swift-kim]

There are a few more "Channel" implementations that depend on FlutterTizenEngine, and I'm planning to refactor them as well after this change.

[bbrto21]

It looks similar to the Android implementation and In my view, this is better.

[pkosko]

LGTM

[bwikbs]

It looks similar to the Android implementation and In my view, this is

@bbrto21 Why do you think so? I'm just curious... I want to know even a small or minor part.

[bbrto21]

It looks similar to the Android implementation and In my view, this is

@bbrto21 Why do you think so? I'm just curious... I want to know even a small or minor part.

I've just seen these
https://github.com/flutter/engine/blob/6a87e0b558e45ae0e7d3369757512bb3c7c151fa/shell/platform/android/io/flutter/plugin/common/StringCodec.java#L17
https://github.com/flutter/engine/blob/6a87e0b558e45ae0e7d3369757512bb3c7c151fa/shell/platform/android/io/flutter/embedding/engine/systemchannels/LifecycleChannel.java#L20

[swift-kim]

@bbrto21 FYI, it's also consistent with the Dart side definition of the lifecycle channel.

https://github.com/flutter/flutter/blob/2.2.1/packages/flutter/lib/src/services/system_channels.dart#L210-L223