greeter:fixed the wrong of is_sessioned_on in accounts.coffee

The bugs caused:
1.user switch menu auto show
2.desktop switch menu cannot show

tower:https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/a318654bb20f4bc0b0915b0b7c11444f/#835bb459c6b847fcaa0d2ebeb5a4350e

Change-Id: If048243ed210abdb7f18a647b2a45e2ead14804e