Provide a way to specify a polling mode for neard

The re-introduced neard NFC backend now starts target detection in the "Initiator" mode.

See this commit: https://codereview.qt-project.org/c/qt/qtconnectivity/+/531986

However, the adapter generally supports 3 different modes, so we should provide a possibility to change the polling mode.

As Qt NFC does not provide any way to set backend-specific parameters, the easiest way to do it would be to introduce an environment variable.