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.
Gradle projects are evaluated in lexicographical order, and the plugin
projects are at the same level as the :app project, so if a plugin has
a name that comes before 'app' (like, for example, any name that starts
with a capital letter), the plugin project will be evaluated before
:app.
Since :app applies the Flutter Gradle plugin, which tries to
modify the dependencies of the plugin projects, we have a problem if the
plugin projects have already been evaluated. Adding
evaluationDependsOn(':app') to the plugin projects fixes this.
Updated example projects to the latest (plugin-enabled) Gradle build
files.
Also removed two unused imports in `pluginClass.java.tmpl`.
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).
