feat: support android badge include oreo+ version

[beygee]

Hi. Android Oreo+ officially support App Badge. But it needs NotificationChannel.

Features

• Support Android Oreo+ App Badge
• Migration embedding v2 Flutter Example

IMPORTANT

In Oreo+ Version, it need ic_launcher.png app logo file in android/app/src/main/res/drawable folder. Because of NotificationChannel's Noti must need app logo file.

References

• https://developer.android.com/training/notify-user/badges#java
• https://developer.android.com/training/notify-user/channels
• https://samse.tistory.com/entry/Android-Badge-count

[beygee]

@g123k

[g123k]

I've already explained that I don't want to generate a notification on Oreo+ devices. Please fork this repo instead