Try not to always return to MessageList

Switched MessageList from singleTask to singleInstance launchMode
http://stackoverflow.com/questions/2417468/android-bug-in-launchmode-singletask-activity-stack-not-preserved

This makes launched activities to initiate a new task, they have to
handle the BACK key if user has the option enabled. On the other hand,
K-9 still keeps a single instance of MessageList (as opposed to using
the default launch mode which would allow multiple instances -
potential increased memory usage).

See Issue 2467