Add logind integration to screenlocker

A new class LogindIntegration is added to the screenlocker. This class
listens to the Lock and Unlock signals emitted by the session object on
the logind service.

This is integrated with the screenlocker application to start or quit the
lock when the signals are emitted. This means a locked screen is unlocked
when the Unlock signal is received from logind. Logind is a higher
authority for the screenlocker and thus follows the requests from logind.


REVIEW: 117355