Redefine FLAG_ACTIVITY_NEW_DOCUMENT without NEW_TASK

Change definition of FLAG_ACTIVITY_NEW_DOCUMENT from
FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET to
FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET alone.

Also add new documentLaunchMode of "never" to allow activity
writers to keep their activity from being launched in document mode.

Fixes bug 15468528.

Change-Id: Ied11adf97e85c5d3f99f4c0bbbb4a2905dcfb24e