title: Authentification in Android App without user login
body: <p>I am trying to implement a dialogsystem (Google Dialogflow CX) into an Android App. I am trying to connect via REST. Appparently, CX doesn't support API keys and the library commonly used is not supported in Android either, so I am facing problems.</p>
  <p>With the credentials available in raw (implementation as previously used with dialogflow ES), I get the following error message:</p>
  <pre><code>07-09 12:10:27.841 2600-2600/com.softbankrobotics.jokeswithdialogflow E/AndroidRuntime: FATAL EXCEPTION: main
      Process: com.softbankrobotics.jokeswithdialogflow, PID: 2600
      java.lang.NoClassDefFoundError: Failed resolution of: Ljava/time/Duration;
          at com.google.auth.oauth2.OAuth2Credentials.&lt;clinit&gt;(OAuth2Credentials.java:70)
          at com.google.auth.oauth2.ServiceAccountCredentials.fromStream(ServiceAccountCredentials.java:475)
          at com.softbankrobotics.jokeswithdialogflow.data.DialogflowDataSource.&lt;init&gt;(DialogflowDataSource.kt:17)
          at com.softbankrobotics.jokeswithdialogflow.MainActivity.onCreate(MainActivity.kt:52)
          at android.app.Activity.performCreate(Activity.java:6257)
  ...
  </code></pre>
  <p>It seems the first error refers to the min SDK having to be at 26. Unfortunately, I need 23 for my app to run on pepper robot - so I can't alter this.</p>
  <p>I would like to authenticate once (e.g. with my own Google account), with validity for all the test users. Safety aspects can be disregarded here, as the implementation is intended for testing in a scientific study, only.</p>
  <p>Thank you in advance - any help is appreciated!</p>