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.c

@@ -0,0 +1,16 @@
+#include "device.h"
+
+SDL_bool device_read_info(TCPsocket device_socket, char *device_name, struct size *size) {
+    unsigned char buf[DEVICE_NAME_FIELD_LENGTH + 4];
+    if (SDLNet_TCP_Recv(device_socket, buf, sizeof(buf)) <= 0) {
+        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Could not retrieve device information");
+        return SDL_FALSE;
+    }
+    buf[DEVICE_NAME_FIELD_LENGTH - 1] = '\0'; // in case the client sends garbage
+    // strcpy is safe here, since name contains at least DEVICE_NAME_FIELD_LENGTH bytes
+    // and strlen(buf) < DEVICE_NAME_FIELD_LENGTH
+    strcpy(device_name, (char *) buf);
+    size->width = (buf[DEVICE_NAME_FIELD_LENGTH] << 8) | buf[DEVICE_NAME_FIELD_LENGTH + 1];
+    size->height = (buf[DEVICE_NAME_FIELD_LENGTH + 2] << 8) | buf[DEVICE_NAME_FIELD_LENGTH + 3];
+    return SDL_TRUE;
+}