Port notifications to method channel.

[ManasviPatidar]

Currently we use the flutter_local_notification plugin to set contest notifications. We can improve upon the efficiency and reduce the weight of our application by writing our custom method channel.
This will include:-

• boilerplate code for communication with the platform.
• setNotification()
• getScheduledNotifications()
• cancelNotification()

[BURG3R5]

@praeclarumjj3 I'd like to take this up, whenever we reach this part.