Extract server-related functions

To lighten screen.c, move start_server() and stop_server() to a separate
file.

app/src/server.c

@@ -0,0 +1,21 @@
+#include "command.h"
+
+#include <SDL2/SDL_log.h>
+#include <errno.h>
+
+process_t start_server(const char *serial) {
+    const char *const cmd[] = {
+        "shell",
+        "CLASSPATH=/data/local/tmp/scrcpy-server.jar",
+        "app_process",
+        "/system/bin",
+        "com.genymobile.scrcpy.ScrCpyServer"
+    };
+    return adb_execute(serial, cmd, sizeof(cmd) / sizeof(cmd[0]));
+}
+
+void stop_server(process_t server) {
+    if (!cmd_terminate(server)) {
+        SDL_LogError(SDL_LOG_CATEGORY_SYSTEM, "Could not terminate server: %s", strerror(errno));
+    }
+}