Register Flutter plugins at runtime

[HosseinYousefi]

Add the ability to register any object of type FlutterPlugin and ActivityAware in JniPlugin.

We can then use JniPlugin.registerFlutterPlugin to add an implementation of FlutterPlugin that calls its onAttachedToEngine and other methods at the right time.

Note

This could be further simplified to require something like <T extends FlutterPlugin & ActivityAware> JniPlugin.register(T plugin) however we don't yet support intersection types and implementing multiple interfaces – #1021. Alternatively we could create a interface ActivityAwareFlutterPlugin implements FlutterPlugin, ActivityAware and include it.

We might decide to expose the bindings in another package, for example package:flutter_jni, however the Java code must still reside in package:jni until we stop depending on Flutter and use the native assets system – #645.