monitor-manager: Make sure to emit PropertiesChanged for PowerSaveMode

We overrode the property for PowerSaveMode, which meant that gdbus's
auto-generated PropertiesChanged code wasn't being run.

This really confused gnome-rr and gnome-settings-daemon's power plugin
about the current DPMS state of the display, since they used their
cached PowerSaveMode properties, and never saw a PropertiesChanged being
emitted.

If a display was on, they set it to off, and then set it back on, the
setting back on would never fire, since they thought the display was
already off.

To fix this, remove our custom property override and just respond to
notifications on the object.

Namely, this fixes the DPMS management when receiving notifications so
that it now properly times out.