To to test the return value of a method inside a method using Mockito in andorid

Question

I'm new to Unit testing concept, and would like to test the following scenario.

public interface CredentialService{
    String getAuthToken();
}

Following implements the service

public class CredentialImpl implements CredentialService{
     @Override
     public String getAuthToken(){
        if(StorageManager.isExpired()) {
          return StorageManager.getToken();
        } else {
          return "Some error";
        }
}

Storage logic it's a singleton class

public class  StorageManager{

  public void setToken(String token) {
    //stores into the prefs
  }

  public String getToken(){
    //get token from prefs
  }

  public boolean isExpired(){
     return false;
  }

}

Here I would like to test CredentialImplTest and following is my test class

@Config(constants = BuildConfig.class, sdk = TestConfig.SDK)
@RunWith(RobolectricGradleTestRunner.class)
public class CredentialImplTest extends TestCase {
    CredentialServiceImpl mSubject = new CredentialServiceImpl();
    StorageManager mStorageManager = mock(StorageManager.class);
    String mTestToken = "abcd";

@Test
public void getAccessToken_noError_expectToken(){
    mStorageManager.setAccessToken(mTestToken);
    when(mSubject.getAuthToken())
            .thenReturn(mTokenUtils.getAccessToken());
    assertEquals(mTestToken, mSubject.getAccessToken());
}
}

If I try to execute the above test the Android Studio throwing E.g. thenReturn() may be missing.

Examples of correct stubbing:
    when(mock.isOk()).thenReturn(true);
    when(mock.isOk()).thenThrow(exception);
    doThrow(exception).when(mock).someVoidMethod();
Hints:
 1. missing thenReturn()
 2. you are trying to stub a final method, you naughty developer!

Is my approach to test getAuthToken() method right, and if not please suggest me the right one?