Avoid ConversationList/ComposeMessage activity leaks

First, ConversationList activity leak, Contact.mListeners are only unbound
when ListItemViews are recycled, without considering the activity exit case
or message delete case.
Second, ComposeMessage activity leak: use SoftReferences instead of
HardReferences for the slideshow cache.

Change-Id: Ic235b31403b26aba16bf6b8b51bff7b4c6f68d98
Signed-off-by: Guobin Zhang <guobin.zhang@intel.com>
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>