Raspberry Pi, SSR, Solenoid Valve Power (Multiplexing?)
The raspberry Pi cannot be connected through a resistor to AC mains. One because it is AC, and the Pi requires DC. Secondly, because it is not safe (AC mains should be isolated). Thirdly, because there is no regulation.
By regulation, the Pi is a variable power load, the 2.5A is a max spec, so sometimes it might need less than 2.5A. Lets say it needs 1A then you would need a 115V/1A = 115Ω resistor (or if you had a fixed resistor of 46Ω then it would only drop 55V and the pi would be exposed to more than 60V which would fry it)
You need a 120V AC to 5V DC power supply