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/AndroidDevice.tsx

@@ -10,6 +10,7 @@ import {Priority} from 'adbkit-logcat-fb';
 import child_process from 'child_process';
 import child_process_promise from 'child-process-es6-promise';
 import ArchivedDevice from './ArchivedDevice';
+import {ReadStream} from 'fs';
 
 type ADBClient = any;
 
@@ -99,4 +100,18 @@ export default class AndroidDevice extends BaseDevice {
     const shellCommand = `am start ${encodeURI(location)}`;
     this.adb.shell(this.serial, shellCommand);
   }
+
+  screenshot(): Promise<Buffer> {
+    return new Promise((resolve, reject) => {
+      this.adb.screencap(this.serial).then((stream: ReadStream) => {
+        const chunks: Array<Buffer> = [];
+        stream
+          .on('data', (chunk: Buffer) => chunks.push(chunk))
+          .once('end', () => {
+            resolve(Buffer.concat(chunks));
+          })
+          .once('error', reject);
+      });
+    });
+  }
 }