Added a PluginRegistry to the new project template. The registry files will be automatically updated at build time to register the native plugins.

Fixes #7814.

Plugin projects are created by running `flutter create --plugin <name>`.

An example app is also created in the plugin project, using the normal 'create' template, which has been modified to allow for conditional plugin code.

Modified the android package name to match package naming conventions (all lower-case, and must match the directory name).