Fix screen recording bug on Pixel devices

Summary:
Pixels don't seem to allow modifying the modification time
of files on the "sdcard". You can repro this by getting a Pixel
device and running

```
$ adb shell touch /sdcard/boop
$ adb shell touch /sdcard/boop
touch: '/sdcard/boop': Invalid argument
```

Access time seems to work, strangely enough, but I'd rather
not play games and just create an empty file instead with echo.

Reviewed By: danielbuechele

Differential Revision: D13941381

fbshipit-source-id: 1321482a07a6698aa5eaf71e5a022c8e7dfe0b18

src/chrome/ScreenCaptureButtons.js

@@ -166,7 +166,7 @@ class ScreenCaptureButtons extends Component<Props, State> {
 
       this.executeShell(
         selectedDevice,
-        `mkdir -p "${devicePath}" && touch "${devicePath}/.nomedia"`,
+        `mkdir -p "${devicePath}" && echo -n > "${devicePath}/.nomedia"`,
       )
         .then(output => {
           if (output) {