Cleanup and some code reuse between Device- and normal Plugins

Summary: Introducing a base abstract class (blegh) to share some life cycle management between Device- and normal plugins. Cleaned up the test utils a bit as well + some old TODO's that now have been taken care of

Reviewed By: nikoant

Differential Revision: D22727089

fbshipit-source-id: 507816f1bfdbc6e7e71d4bd365b881b6710ca917

desktop/flipper-plugin/src/state/atom.tsx

@@ -9,7 +9,7 @@
 
 import {produce} from 'immer';
 import {useState, useEffect} from 'react';
-import {getCurrentPluginInstance} from '../plugin/Plugin';
+import {getCurrentPluginInstance} from '../plugin/PluginBase';
 
 export type Atom<T> = {
   get(): T;