Modem technology preference is not reset on flash+wipe

Test case on mako HW, image 186:
 * Open System settings / Cellular
 * Set mobile data to "2G" (corresponds to oFono property RadioSettings.TechnologyPreference)
 * (Optionally, check that this setting is preserved over reboot)
 * Flash a new software image using 'ubuntu-device-flash --wipe'

Expected result: All system settings, including the 2G/3G preference, are reset to the default

Actual result: The mobile data is still "2G

I think the reason is that the technology preference is stored only in the modem firmware's own non-volatile memory, which is not touched in an Ubuntu update. To fix this, some component (oFono or one of its clients) would need to set the default "3G" in first boot.

Note: I only tested this on mako (with a Qualcomm modem), other modems might behave differently.