Add a logging sink for Android

[sithhell]

Android provides a special logging facility. It would be very helpful hpx could log to that directly.

The API to implement is this:

#if defined(ANDROID) || defined(__ANDROID__)

#include <android/log.h>

// ...
const char * some_text = ...
__android_log_write(ANDROID_LOG_DEBUG, "hpx", some_text);

#endif