So I've been trying to do the same thing, and just discovered something when I'm trying to launch the Assistant with an Intent like this:
Intent launchIntent = new Intent(Intent.ACTION_VIEW);
launchIntent.setPackage("com.google.android.googlequicksearchbox");
launchIntent.setClassName("com.google.android.googlequicksearchbox", "com.google.android.apps.gsa.staticplugins.opa.OpaActivity");
startActivity(launchIntent);
Although this doesn't throw an ActivityNotFoundException, it does throw a Permission Denial. This is shown to happen because Google Assistant must be launched from the googlequicksearchbox package from above. Hopefully this will change when/if they release an API for it. Fingers crossed.