Fixed build of example code to use new binary messenger API.

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.

Make platform_channels_swift use more idiomatic Swift

* Remove semicolons.
* `if (condition)` => `if condition`.
* `Class.init(...)` => `Class(...)`.
* Remove `break` statements from `switch` cases.
* Remove some unnecessary uses of `self`.

* wip

* wip

* delete main.m

* readd main.m

* update to use new channel api

* update android

* removed android/

* remove debug print

* remomved main.m

* Update year in copyright

* break long line and update name

* mit comments

* update examples/README

* break line

* update README

* update test