fix groupsActivity not being properly launched from solo mode using the menu.
prevent user to go from a group to single user mode
improve app stability. It should be no longer possible to have two instances of the activity running at the same time. This has been tested using NFC only. Note that when starting the loginActivity from another point (like we do in AutoJoin), you should add a clearTop flag so that if the user was already in the loginActivity, it won't be duplicated