This is really strange behavior, because there is no dialog about anything beeing erased at device admin; only there are informations about application´s possibilities (f.e. can delete messages, can delete contacts, etc.) If you uncheck the antitheft there, you should see page with Deactivate a Cancel buttons and deactivate will only uncheck the applications in list. Unchecking application in device admin will not cause anything deleted. You have always choice to check / uncheck the app in device admin, in case you have not, something wrong happen to your android OS (may be try to upgrade on 2.2.2 wil help too). Anyway, beeing device admin have nothing to do with Avast Mobile Security.