Follow up on 68ba2785, which missed the
sad fact that m_aMutex is locked recursively.

Avoid that by passing a ClearableMutexGuard to stopTicker() and
unlocking that.  Also lock m_aMutex in the destructor while at it.

Change-Id: I5ef7ef8f15e2b5c9810c5ffc64ed922ab9ad2807