Try harder to find suitable camera parameters

This branch fixes QR code scanning on the Galaxy Nexus running Cyanogen Mod 12.1 (Android 5.1.1), without breaking QR code scanning on any of the other test devices.

The problem on the Galaxy Nexus was that the selected scene mode was overriding the selected focus mode, so we asked for continuous picture mode but got macro mode. Macro mode requires startAutoFocus() to be called, but we weren't calling it because we'd asked for continuous picture mode.

The fix for that problem is to query the focus mode after applying the parameters and call startAutoFocus() based on the actual mode rather than the requested mode.

But then I discovered another problem: barcode scene mode was setting the flash to auto, so in low light the flash was turning on and off while trying to scan QR codes. That might work well for printed QR codes, but it's terrible when scanning from a screen.

The fix for the new problem is to select barcode scene mode, then try to disable the flash, and if that fails, reset the scene mode. Then we pick the best available video stabilisation, focus mode and preview size.

On the Galaxy Nexus with CM 12.1, that means we use continuous picture mode instead of barcode scene mode, which works fine. All the other test devices pick the same settings as before.