Using an android display with HDMI/RCA inputs? I am currently trying to use the Raspberry Pi to build a small device. Part of this project involves using a screen of some sort, and it needs to be small.  Since Android displays are the right size, it seems like they might have HDMI or RCA inputs because they're commonly used. Is it true these displays have these inputs?
HDMI is a very 'high-level' connection. It doesn't just include a video stream, but also audio, packet data (to allow the TV to control other connected devices), and sometimes even ethernet. It allows screen size, colour, and refresh rate negotiation. The display end of the HDMI connection needs a lot of logic to negotiate the protocol, and understand the video data. All of this is unnecessary overhead for a screen that can only display one format and can only be replaced by an identical screen.

For this reason, the internal connection inside most portable devices (including Android phones, tablets, development boards, etc.) is a much lower-level connection, often DSI. Important hardware such as the framebuffer controller is on the system side of the DSI interface, not the screen side. It would be a lot of work to allow a Pi to speak on a DSI interface, and the work would be specific to the particular screen you use. You'd basically be making all of a computer monitor except the screen.