Add dispose function for proper disabling of tracking

[Robert27]

Executing init with an empty key string leads to the following warning Tracking will be disabled.

However, this tracking is only deactivated if there is no previously initialized tracking object.
Otherwise, the old one will continue to be used, although the warning promises that tracking is now disabled.

Specifically, we use it to deactivate tracking as a user option during the app runtime. The console warning implies that tracking is now also off, but the network logs showed that it was not actually deactivated. Thus, it poses an active risk of violating privacy.

This PR fixes this problem by actually stopping the polling in addition and overwriting the old client with undefined.

[cristipufu]

I think it would be more elegant to add a stop/dispose/destroy method. Once you know the user preference, the next time he opens the app, you can simply skip the init call if tracking is disabled

[Robert27]

I have now added a dispose function that can be called directly by the user to disable tracking completely instead of having to initialize it with invalid data.
In addition, this function is still executed in the case of an incorrect init in order to comply with the console warning and to prevent incorrectness. Although it is checked a second time whether the client exists at all, this is the simplest solution for me to avoid generating a false warning.