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/utils/screenshot.tsx

@@ -0,0 +1,31 @@
+/**
+ * Copyright 2018-present Facebook.
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ * @format
+ */
+
+import fs from 'fs';
+
+/**
+ * Writes a buffer to a specified file path.
+ * Returns a Promise which resolves to the file path.
+ */
+export const writeBufferToFile = (filePath: string, buffer: Buffer) => {
+  return new Promise((resolve, reject) => {
+    fs.writeFile(filePath, buffer, err => {
+      if (err) {
+        reject(err);
+      } else {
+        resolve(filePath);
+      }
+    });
+  });
+};
+
+/**
+ * Creates a Blob from a Buffer
+ */
+export const bufferToBlob = (buffer: Buffer): Blob => {
+  return new Blob([buffer]);
+};