Wrap SDL thread functions into scrcpy-specific API

The goal is to expose a consistent API for system tools, and paves the
way to make the "core" independant of SDL in the future.

app/src/input_manager.c

@@ -4,7 +4,6 @@
 #include <SDL2/SDL_keycode.h>
 
 #include "event_converter.h"
-#include "util/lock.h"
 #include "util/log.h"
 
 static const int ACTION_DOWN = 1;