Make client.device fulfill type constraints

Summary:
This is an interesting invariant that TS caught here. We
expect `getDevice()` to always return a device but it
cannot because it's set lazily. John Knox suggested we instead
set up a promise in the constructor and resolve it instead of
overriding the stateful promise later.

Reviewed By: jknoxville

Differential Revision: D17313468

fbshipit-source-id: 8fd75f2720546abf67beead23db56216f1a5e0df

src/plugin.tsx

@@ -120,7 +120,7 @@ export abstract class FlipperBasePlugin<
   reducers: {
     [actionName: string]: (state: State, actionData: any) => Partial<State>;
   } = {};
-  app: App;
+  app: App | null = null;
   onKeyboardAction: ((action: string) => void) | undefined;
 
   toJSON() {