Add android enable/disable to settings screen

Summary:
The UI could do with a bit more fine tuning, but it's a start.

Fully functional with updating the settings file. But nothing actually uses that setting yet.

Reviewed By: priteshrnandgaonkar

Differential Revision: D17810588

fbshipit-source-id: 791ee60616f3ee73f41813a5a442b08a5e395458

src/reducers/settings.tsx

@@ -9,6 +9,7 @@ import {Actions} from './index';
 
 export type Settings = {
   androidHome: string;
+  enableAndroid: boolean;
 };
 
 export type Action =
@@ -20,6 +21,7 @@ export type Action =
 
 const initialState: Settings = {
   androidHome: '/opt/android_sdk',
+  enableAndroid: true,
 };
 
 export default function reducer(