Modified code in consideration of API

It has been modified from finishAndRemoveTask() to finish() in consideration of API  < 21.

src/main/java/com/owncloud/android/ui/activity/PassCodeActivity.java

@@ -367,7 +367,7 @@ public class PassCodeActivity extends AppCompatActivity implements Injectable {
         if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount()== 0){
             if(ACTION_CHECK.equals(getIntent().getAction())){
                 moveTaskToBack(true);
-                finishAndRemoveTask();
+                finish();
             }else if (ACTION_REQUEST_WITH_RESULT.equals(getIntent().getAction()) ||
                 ACTION_CHECK_WITH_RESULT.equals(getIntent().getAction())) {
                 finish();