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/PluginContainer.tsx

@@ -142,6 +142,7 @@ class PluginContainer extends PureComponent<Props, State> {
       | null
       | undefined,
   ) => {
+    // N.B. for Sandy plugins this lifecycle is managed by PluginRenderer
     if (this.plugin) {
       this.plugin._teardown();
       this.plugin = null;