How is the Angle of Attack influenced in the slipstream of a propeller?
Not all planes have propellers and not all of the wing is behind the propeller. 

In fact only a small part of the wing is behind the propeller. The rest will get clean air depending on the attitude (orientation) of the aircraft. The wing is also not perfectly straight behind the propeller either, usually dropping down a bit so there is a positive angle of attack.

Jet powered planes generally have their engines away from the wings.

In most planes, thrust is surprisingly small compared to the weight, an empty A380 only has a thrust to weight ratio of 0.47 and most of that thrust will be applied horizontally. So as the plane will sink rapidly should there be no lift from the wings. That sink tendency is what provides the angle of attack. 

This will max out the sink rate should the wings be level, angle them up a bit and the plane will try to climb.