Android: use number of available processors

_ONLN is the currently online number of processors, while _CONF
is the number of configured processors, some of which can be disabled.

src/android/thread.c

@@ -417,5 +417,5 @@ mtime_t mdate (void)
 
 unsigned vlc_GetCPUCount(void)
 {
-    return sysconf(_SC_NPROCESSORS_CONF);
+    return sysconf(_SC_NPROCESSORS_ONLN);
 }