Use string message codec for lifecycle plugin

[vially]

The lifecycle plugin was using a standard message codec but it is supposed to be using a string message codec.

I haven't found any official documentation about why that's the case (except for these code comments) but that's how it is currently implemented for both iOS and Android.

Without this change, neither the engine nor the dart side were able to correctly decode the lifecycle messages.

This pull-request fixes that by adding a StringMessageCodec implementation and replacing the StandardMessageCodec with it in the lifecycle plugin.

PS: I'm okay with delegating all rights related to this change to Sony.

[HidenoriMatsubayashi]

Thanks for PR.

Can you please update https://github.com/sony/flutter-embedded-linux/blob/master/AUTHORS and sign a DCO (Developer Certificate of Origin) in git comment?

[vially]

Can you please update https://github.com/sony/flutter-embedded-linux/blob/master/AUTHORS

Should be fixed now.

... and sign a DCO (Developer Certificate of Origin) in git comment

I think this should have been signed already. Is there anything else that I need to do except for the the Signed-off-by: ... git commit message?

[HidenoriMatsubayashi]

I think this should have been signed already. Is there anything else that I need to do except for the the Signed-off-by: ... git commit message?

Oh, I was wrong. Thanks.

Can you please format the source files to fix CI checks?

[HidenoriMatsubayashi]

Can you please format the source files to fix CI checks?

It looks like it's not your change. I'll fix this errors. Wait a second, please.

[HidenoriMatsubayashi]

Fixed CI errors. Can you please rebase?

[vially]

Fixed CI errors. Can you please rebase?

Done. Please try again now.

[HidenoriMatsubayashi]

@vially Thanks a lot!