Apps can launch other apps' activities inside their task. This makes it impossible to switch back and forth between the two. This module sets the FLAG_ACTIVITY_NEW_TASK flag when starting a new activity, which 'fixes' that (note to devs: only startActivity is hooked for obvious reasons.) Note that this module may break things. See the XDA thread for more info.
http://repo.xposed.info/module/com.germainz.activityforcenewtask
