[google_maps_flutter_web] Stop listening to map events when disposed

[JamesMcIntosh]

Fixes flutter/flutter#168782

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[mdebbar]

I don't think we need this if (!_streamController.isClosed) check here and below.

The _onClickSubscription will be canceled as soon as the maps controller is disposed. That means the listener will not be called, so there's no need to do this check.

The only exception is map.onTilesloaded.first since that's a future and can't be "canceled".