DON-386: Add accessibility support to BPKNudger

[yurareutskiy]

In the scope of improving feature tests of the traveller selector there is essential change for BPKNudger, which is using as a stepper component in TravellerSelector.

The problem is some small components like increment and decrement buttons are unavailable for clicking in UITests due to combining those elements into one Nudger accessibility element.

Those changes make smaller elements accessible without any changes for screen reader for actual accessibility users.

• Check this if you have read and followed the contributing guidelines

Remember to include the following changes:

• README.md
• Tests
• Screenshotting code
• Adding a component? Remember to expose it in the main Backpack.h header file

If you are curious about how we review, please read through the code review guidelines

[frugoman]

Looks good!