Introduce onConnect / onDisconnect hooks

Summary:
Introduced hooks that are called whenever the plugin is connected / disconnected to it's counter part on the device.

There is some logic duplication between `PluginContainer` for old plugins, and `PluginRenderer` for new plugins, mostly caused by the fact that those lifecycles are triggered from the UI rather than from the reducers, but I figured refactoring that to be too risky.

Reviewed By: jknoxville

Differential Revision: D22232337

fbshipit-source-id: a384c45731a4c8d9b8b532a83e2becf49ce807c2

desktop/app/src/plugin.tsx

@@ -297,10 +297,7 @@ export class FlipperPlugin<
     }
     // run plugin teardown
     this.teardown();
-    if (
-      this.realClient.connected &&
-      !this.realClient.isBackgroundPlugin(pluginId)
-    ) {
+    if (!this.realClient.isBackgroundPlugin(pluginId)) {
       this.realClient.deinitPlugin(pluginId);
     }
   }