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/flipper-plugin/src/plugin/PluginContext.tsx

@@ -8,16 +8,8 @@
  */
 
 import {createContext} from 'react';
+import {SandyPluginInstance} from './Plugin';
 
-export type SandyPluginContext = {
-  deactivate(): void;
-};
-
-// TODO: to be filled in later with testing and such
-const stubPluginContext: SandyPluginContext = {
-  deactivate() {},
-};
-
-export const SandyPluginContext = createContext<SandyPluginContext>(
-  stubPluginContext,
-);
+export const SandyPluginContext = createContext<
+  SandyPluginInstance | undefined
+>(undefined);