How to Make Android phone Slave Display Raspberry Pi 3b? I would like to have an extra display always with me. The only thing which I have most often with me is my Phone with CM13. 
Phone


Oneplus 3 and its USB Type C connection may be suitable for the task. 
Software: make CM13 software understand that I want to use it as a slave for other microprocessor.  


Client RP3b 


There is HDMI signal from RP3b. I can use usbC-to-HDMI adapter but not sure if it is needed because the data rate is sufficient in USB-C itself. 


Phone: Oneplus 3
System: Cyanogenmod 13 Stable
Phones failed: Samsung Galaxy S4 gt-i9506   

Phone: Oneplus 3T
OS: Oreo 8.0
This feature needs to be supported by the phone manufacturer. 
The software is not a challenge after that. 
I know no phone which has such a feature at the moment.