Use Log.getStackTraceString in JULHandle example

the previously used implementation to print the stacktrace would only
print the stacktrace of the resulting Exception/Throwable, but the
really relevant stacktrace is the one from the causing
Exception/Throwable. Androids android.util.Log.getStackTraceString()
achieves this by

while (t != null) {
    t = t.getCause();
}