Fix crashes that occurs because AsyncTasks are still running after stopping a survey (by invoking wootric.stop())
The AsyncTasks are invoking onSurveyValidated(), onAuthenticateSuccess(), onGetEndUserIdSuccess(), onEndUserNotFound() and onCreateEndUserSuccess() which therefore needs checks to cancel if the survey has been stopped. Fixes https://github.com/Wootric/WootricSDK-Android/issues/126
Based of https://github.com/Wootric/WootricSDK-Android/pull/135