It is not very clear what exactly you are trying to achieve, but I hope following will be helpful.
The code given at How to programmatically initiate a Google Now voice search? will launch the default speech recognizer (or "voice assistant" as you have put it).
Following, however, will explicitly open (if available) Google's speech recognizer:
Intent intent = new Intent(Intent.ACTION_MAIN);
intent.setClassName("com.google.android.googlequicksearchbox",
"com.google.android.googlequicksearchbox.VoiceSearchActivity");
try {
startActivity(intent);
} catch (ActivityNotFoundException anfe) {
Log.d(TAG, "Google Voice Search is not found");
}