Moved screenshot functions into Device's class.

Summary:
I have moved the screenshot functions from ScreenCaptureButtons to the Device classes. I have slightly rewritten them so that they return a Promise which resolves to a Buffer. The Buffer can then be saved to a file or converted to a data Blob.

I have removed streaming and simply loaded the image into memory. Once the image is in memory it can be manipulated for various tasks i.e. written to a file, or displayed in the app.

iOS screenshots had to be rewritten. I now save the image to a temp folder, load it into the apps memory, and then remove the temp image.

Reviewed By: jknoxville

Differential Revision: D16939901

fbshipit-source-id: 3e39a5aeda8d48829ac5a8ff912a98f110341c07

src/devices/BaseDevice.tsx

@@ -145,4 +145,10 @@ export default class BaseDevice {
   archive(): any | null | undefined {
     return null;
   }
+
+  screenshot(): Promise<Buffer> {
+    return Promise.reject(
+      new Error('No screenshot support for current device'),
+    );
+  }
 }