QR Code Scanner Service

[chebizarro]

Create a new service which actively scans for QR codes as the user walks or on demand.
The currently used QR code library is journeyapps/zxing-android-embedded.
The branch qrs currently has the QR scanner launched from the main activity.

[chebizarro]

The RxBarcodeScanner repo has a first implementation of a barcode scanner that uses the Camera2 API which doesn't require a preview. Specific issues related to this functionality should be submitted here.