droidcamsrc: reworked RoI handling
droidcamsrc: reworked RoI handling

- we now support setting focus and metering areas but default to both if no type has been specified
- take into account the max number of focus and metering areas supported by the HW

sailfishos/gst-droid,mlehtima/gst-droid,foolab/gst-droid,foolab/gst-droid,mlehtima/gst-droid