ot::AnnounceSenderBase::SendAnnounce() method

Schedules the MLE Announce transmissions. Requests @p aCount additional MLE transmission cycles to be scheduled. Each cycle covers all the channels in the specified channel mask from `GetChannelMask()`, with `GetPeriod()` as the time interval between any two successive MLE Announcement transmissions (possibly) on different channels from the mask. The `GetJitter()` value is used to add a random interval from `[-jitter, jitter]` to each period interval. If a "starting channel" is specified using `SetStartingChannel()`, then the cycle starts with MLE Announce transmission on the given starting channel. Otherwise the cycle starts with the first channel (with smallest channel number) in the mask. If a previously requested MLE Announce transmission is still ongoing, a subsequent call to this method adds the @p aCount cycles to the schedule. If there is no active MLE Announce transmission, after call to `SendAnnounce()`, the first MLE Announce transmission happens within a short random interval selected from range `[0, jitter]`.