Returns a valid instance of
MarketingCloudSdk when it is ready. If initialization of the SDK has already completed then
ready will be called immediately. If
init has not been called, or is executing,
ready will be called upon completion. Note: If
init completes with a status of
#ready(MarketingCloudSdk) ready will not be called, and the current list of
WhenReadyListener listeners will be cleared.