Add docs to SystemClock.h

#include <stdint.h>

#include <sys/types.h>

// See https://developer.android.com/reference/android/os/SystemClock

// to learn more about Android's timekeeping facilities.

namespace android {

// Returns milliseconds since boot, not counting time spent in deep sleep.

int64_t uptimeMillis();

// Returns nanoseconds since boot, not counting time spent in deep sleep.

int64_t uptimeNanos();

// Returns milliseconds since boot, including time spent in sleep.

int64_t elapsedRealtime();

// Returns nanoseconds since boot, including time spent in sleep.

int64_t elapsedRealtimeNano();

}  // namespace android