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: I69e9e264e8cd8d37df5d032eb6f9447afbc3ff4d
Signed-off-by: Guobin Zhang <guobin.zhang@intel.com>