Stop previous TaskStack worker

Each time we delete all threads, a new TaskStack thread is started.
We need stop the previous one to avoid over memory consumption.

Change-Id: Ibee8dd7b8e757df0686c7989dc7d9878f9ef5102
Signed-off-by: Emmanuel Berthier <emmanuel.berthier@intel.com>
Merge "Stop previous TaskStack worker"
auto-merger: merge in from aosp/master