Finally, I could able to retrieve the auth_token through this snippet:
final AccountManagerFuture<Bundle> future = AccountManager.get(this)
.getAuthToken(account, authTokenType, null, this, null, null);
new Thread(new Runnable() {
@Override
public void run() {
try {
Bundle bnd = future.getResult();
final String authtoken = bnd
.getString(AccountManager.KEY_AUTHTOKEN);
Logging.d(LOG_TAG, (authtoken != null) ? "SUCCESS!\ntoken: "
+ authtoken : "FAIL");
} catch (Exception e) {
e.printStackTrace(); }
}
}).start();