Added multiple select to Choose Contact Activity

When passing the Choose Contact Activity a boolean called "multiple"
set to true, it now allows the user to select multiple contacts. Long
pressing a contact enters multiple selection mode and the selection is
confirmed using a button in the CAB. The Activity returns whether or not
multiple contacts were selected using the same boolean. If multiple
contacts were selected, an array of JID strings called "contacts" is
returned. The Choose Contact Activity now also honors "filter_contacts",
an array of strings containing JIDs that should not be displayed in
the picker.

The invite to conference dialog now uses this feature to enable inviting
multiple contacts at once. Additionally, Invite Contact uses
"filter_contacts" to only display contacts that are not yet
participating in the conversation