Detect volume down key combo for safe mode

It is possible that a module is breaking the device so bad that zygote
cannot even be started. In this case, system_server cannot start and
detect the safe mode key combo, set the persist property, and reboot.

Also on old Android versions, the system directly goes to safe mode
after detecting a key combo without rebooting, defeating the purpose of
Magisk's safe mode protection if we only check for the persist property.

Directly adding key combo check natively in magiskd allows us to enter
Magisk safe mode before the system is even aware of it.

native/jni/utils/xwrap.hpp

@@ -59,4 +59,5 @@ pid_t xfork();
 int xpoll(struct pollfd *fds, nfds_t nfds, int timeout);
 int xinotify_init1(int flags);
 char *xrealpath(const char *path, char *resolved_path);
+int xmknod(const char *pathname, mode_t mode, dev_t dev);