Simplfiy controller, expose scanDataStream

- MethodChannel is now handled inside the controller
- Removed controller init, pass globalKey as required parameter
- Controller now returns a stream of data
- Updated README.md + example

Fix for Android flash toggle

- the QrViewController has a getter that returns a Stream of data that gets updated on every scan result
  - theres no need to init the QrViewController through a method, because the GlobalKey is generated inside the QrViewWidget
  - a dispose method was added to correctly dispose the stream

Add ability to pause/resume the camera

fixed override onMethodCall to adhere interface

* remove iOS test

* Update azure-pipelines.yml for Azure Pipelines

[skip ci]

Added iOS Instructions

This reverts commit e69f268fca.

included swift 5 in the project's specifications and updated androidX dependency

preview stretching after change screen orientation fix

fix bugs

but need to fix preview stretching after change screen orientation

fix #2 #3 and optimization