Power is the product of force and velocity which are both measured directly by power meters like the SRM/Quarq/Powertap/Stages/etc. Those power meter measure force via strain gauges. They multiply measured RPM by 2 pi times the strain gauges' distance to their center of rotation to get velocity. One multiplication returns instantaneous power.

This smart phone application can measure position (via GPS) and acceleration (via accelerometer) from which it can derive velocity.

The big forces on cyclists are the component of gravitational force parallel to the road (zero where it's flat), aerodynamic, and rolling resistance. While the smart phone knows what direction you're headed and where gravity is due to the accelerometer so it can arrive at that force based on your weight, it can't measure anything to do with aerodynamic forces or rolling resistance so it can't come up with an accurate total force and therefore power.

Given actual knowledge of CdA at a variety of angles, Crr, wind speed, and wind direction it could calculate those but it's limited to approximations which aren't enough.