Add android notification channel name

[AAkira]

Currently, Android notification channel names are fixed.
It can now be passed as optional.

• IncomingCallNotificationChannelName
• MissedCallNotificationChannelName

[hiennguyen92]

please use name default in native code. to make sure it always works as before without it

[AAkira]

Using name default is fine.
But the notification channel name cannot contain an empty string, so we need to check an empty string instead of a null check.
I think the null check is simple.

Which is better?