[AC-19] user can control flash from QR scanner

[detlog]

User Story
As an application user i want to switch the flash (LED) on and off by clicking on the flash icon in the QR scanner.

Acceptance Criteria

• default state is off
• user can switch the flash on by clicking the icon, the icon changes from flash off to flash on
• user can turn the flash off by clicking the icon again, icon changes from flash on to flash off
• flash should react accordingly

Technical Specifications
for mockup see Mobile Application left screen
the icons are the material icons
flash_on and flash_off

Dependencies
is blocked by AC-17