Replace all manual pluginKey parsing with a utility

Summary:
Ok this diff got a bit bigger than expected, but I think it makes it easier to understand what "plugin keys" are, and makes them less prone to error.

Previously pluginKeys were composed of: clientId#pluginName, where clientId was itself: app#os#device#device_id

But also, there were some plugin keys where the clientId was a device_id.

Now you deconstruct a plugin key, and will get a tagged object with type: 'device' or 'client', and the properties that they each have.

There is now no custom parsing of these afaik, let's keep it that way.

Since it took me a while to figure out what all these IDs are, I've documented it a bit in clientUtils.

Reviewed By: passy

Differential Revision: D18811848

fbshipit-source-id: eed2e2b5eedafb9e27900dbcf79a389fcaffae95

src/reducers/pluginStates.tsx

@@ -8,6 +8,7 @@
  */
 
 import {Actions} from '.';
+import {deconstructPluginKey} from '../utils/clientUtils';
 
 export type State = {
   [pluginKey: string]: Object;
@@ -53,8 +54,9 @@ export default function reducer(
     return Object.keys(state).reduce((newState: State, pluginKey) => {
       // Only add the pluginState, if its from a plugin other than the one that
       // was removed. pluginKeys are in the form of ${clientID}#${pluginID}.
-      const clientId = pluginKey.slice(0, pluginKey.lastIndexOf('#'));
-      const pluginId = pluginKey.split('#').pop();
+      const plugin = deconstructPluginKey(pluginKey);
+      const clientId = plugin.client;
+      const pluginId = plugin.pluginName;
       if (
         clientId !== payload.clientId ||
         (pluginId && payload.devicePlugins.has(pluginId))