Does ADS-B In receive track or heading angles?
It is the true track angle.


True means with respect to the geographic North (as opposed to the magnetic North)
Track angle means the direction of motion in the horizontal plane (as opposed to heading, which means the direction of the projection of the primary body axis in the horizontal plane). 


Track angle can be derived from a line joining successive position points, but that is not typically how its done. GPS commonly uses a combination of Kalman filtered change of position combined with (Kalman filtered) velocity derived from Doppler shifts. 

For ADS-B In, the track angle is just what is reported by ADS-B Out. It does not involve processing successive position reports.