Move device-related code to device.c

Move the code to read the initial device info from scrcpy.c to a
separate file, device.c.

app/src/device.h

@@ -0,0 +1,14 @@
+#ifndef DEVICE_H
+#define DEVICE_H
+
+#include <SDL2/SDL_net.h>
+#include <SDL2/SDL_stdinc.h>
+
+#include "common.h"
+
+#define DEVICE_NAME_FIELD_LENGTH 64
+
+// name must be at least DEVICE_NAME_FIELD_LENGTH bytes
+SDL_bool device_read_info(TCPsocket device_socket, char *name, struct size *frame_size);
+
+#endif