Extend declarative-camera example with Torch mode

TorchMode implementation for Android (when FFmpeg-backend is used) was prepared in parent of this Task (QTBUG-113154)

It would be nice to have an easy way to test this feature - whether turning on the light works.

It cannot be tested on CI. Cannot be tested on emulator - real device with flashlight is needed.

To make a manual tests possible (maybe automated RTA test in future?) declarative-camera example need to be extended.

Flash options (FlashMode On/Off and TorchMode On/Off) will be:

• visible only in case when feature is available;
• transparent if available but disabled;

Screens with visible options and turned on Torch attached: