Fixes a regression from an unintented change in behavior during the
conversion to an in-app method channel for Android and iOS. Although the
Dart code for their implementations is almost identical to the shared
method channel version, the differences in initialization paths caused
the platform versions to try to use the widget bindings before they had
been set up: The constructor for a `dartPluginClass` is called during
`registerWith`, which is before `main`, but the constructor for the
default implementation isn't called until `CameraPlatform.instance` is
called, since Dart automatically does lazy static class initializtion.

To avoid the issue without forcing bindings to be initialized early,
this makes setting up the platform channel listener lazily.

Fixes https://github.com/flutter/flutter/issues/106236