Improve Swift usage in platform_channel_swift some more (#22318)

Improve Swift usage in platform_channel_swift some more

Make a bunch of changes that willlarche requested in
https://github.com/flutter/flutter/pull/21712:

* Use `static let` within `enum` namespaces for String constants.
* Use `guard` for early exits and to avoid forced unwrapping.
* Explicitly qualify a selector.
* Fix a retain cycle by using a weak reference.