The Inhibit method ignored the value of the "flags" argument and
always sent InterruptSession as inhibition policy. This prevents
sleep, but does not prevent idle actions such as screen locking
and dimming.

This commit changes the requested inhibition policies to match
the documented values for "flags":

* Portals "Suspend" remains PolicyAgent "InterruptSession".
* Portals "Idle" becomes PolicyAgent "ChangeScreenSettings".
* Portals "Logout" and "User Switch" are not supported at this time
  and are merely logged but otherwise ignored.

PowerDevil, which implements the PolicyAgent API, uses the same
policies also to represent the logind "sleep" and "idle" inhibitors,
so we can trust that they behave accordingly.

BUG: 486506
CCBUG: 472541
CCBUG: 335729