Units: migrate code to use Units singleton from LunaNext plugin

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
Test: fix LunaService to match new API for getBootStatus

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
Tests: remove StatusBarServicesConnector stub to work again, now as an instance named statusBarServicesConnector.

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>