Update the full guide to mention that `getFramesRead|Written` are available when using OpenSLES

^{Originally posted by **YouheiLee** November 8, 2021}
At the last of doc "Full Guide To Oboe" said that:
getFramesRead(), getFramesWritten(), getTimestamp() will return Result::ErrorUnimplemented for OpenSLES streams, But I found that in AudioStream.cpp, there has a implementation of getFramesRead():

```
int64_t AudioStream::getFramesRead() {
updateFramesRead();
return mFramesRead;
}
```

and in AudioOutputStreamOpenSLES.cpp, function updateFramesRead are also be implemented.
```
void AudioOutputStreamOpenSLES::updateFramesRead() {
if (usingFIFO()) {
AudioStreamBuffered::updateFramesRead();
} else {
mFramesRead = getFramesProcessedByServer();
}
}
```

does it means those three functions(getFramesRead, getFramesWritten, getTimestamp) is also available in OpenSLES streams before OS version android O？