Avoid getrandom or syscall(SYS_getrandom) in Android __ANDROID_API__ < 28

Android Os older than Android 9.0 (API_LEVEL = 28) broken syscall(SYS_getrandom).

Please, avoid it when building with NDK. Add something like

```
....
#elif defined(__linux__) || defined(__GNU__) || defined(GNU_KFREEBSD)

//---------- ADD THESE CODE --------------
#if defined(SYS_getrandom) && ((!defined(__ANDROID__) || __ANDROID_API__ >= 28)
// use getrandom
return randombytes_linux_randombytes_getrandom(buf, n);
#else
// use /dev/urandom
return randombytes_linux_randombytes_urandom(buf, n);
#endif
//---------------------------------------
#elif defined(BSD) ....
```