Extract server-related functions

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

app/src/screen.c

@@ -16,6 +16,7 @@
 #include "frames.h"
 #include "lockutil.h"
 #include "netutil.h"
+#include "server.h"
 
 #define DEVICE_NAME_FIELD_LENGTH 64
 #define SOCKET_NAME "scrcpy"
@@ -65,23 +66,6 @@ static TCPsocket listen_on_port(Uint16 port) {
     return SDLNet_TCP_Open(&addr);
 }
 
-static 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]));
-}
-
-static void stop_server(process_t server) {
-    if (!cmd_terminate(server)) {
-        SDL_LogError(SDL_LOG_CATEGORY_SYSTEM, "Could not kill: %s", strerror(errno));
-    }
-}
-
 // name must be at least DEVICE_NAME_FIELD_LENGTH bytes
 SDL_bool read_initial_device_info(TCPsocket socket, char *device_name, struct size *size) {
     unsigned char buf[DEVICE_NAME_FIELD_LENGTH + 4];