actions/dimdisplay: Don't dim when screen changes are inhibited

DimDisplay::onIdleTimeout() may not force "Explicit" when triggering
the action manually, but it will still record old screen & keyboard
brightness values regardless of policy and set m_dimmed to true.
We should always do either one or the other.

Hence, track the ChangeScreenSettings policy state within DimDisplay
and use that to skip all of onIdleTimeout() if currently inhibiting.
As per GitLab issue #29, we're better off without "Explicit" here.

Modeled after the DPMS action which has similar policy tracking code.

(cherry picked from commit 2f2ec7c9)