SystemUI: don't add heads up view twice

Recreating the statusbar, followed by a system command call to disable
heads up (via disable flags), could lead to two subsequent calls to
addHeadsUpView(). We had a check to make sure it wasn't added in cm11
but it never got ported. This adds the check back.

Ref: CYNGNOS-1583

Change-Id: I8cfe652e66246c742c532fa6281dd4f30de4721b
Signed-off-by: Roman Birg <roman@cyngn.com>