Use non nullable account in session mix

Signed-off-by: alperozturk <alper_ozturk@proton.me>

app/src/main/java/com/nextcloud/client/account/UserAccountManagerImpl.java

@@ -18,14 +18,12 @@ import android.content.Intent;
 import android.content.SharedPreferences;
 import android.preference.PreferenceManager;
 import android.text.TextUtils;
-import android.util.Log;
 
 import com.nextcloud.common.NextcloudClient;
 import com.owncloud.android.MainApp;
 import com.owncloud.android.R;
 import com.owncloud.android.authentication.AuthenticatorActivity;
 import com.owncloud.android.datamodel.ArbitraryDataProvider;
-import com.owncloud.android.datamodel.ArbitraryDataProviderImpl;
 import com.owncloud.android.datamodel.OCFile;
 import com.owncloud.android.lib.common.OwnCloudAccount;
 import com.owncloud.android.lib.common.OwnCloudClientManagerFactory;
@@ -42,7 +40,6 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Optional;
-import java.util.stream.Stream;
 
 import javax.inject.Inject;