Do not block on <--> dimmed display state transitions due to fading

Delaying transition to or from dimmed state makes the event eater
misbehave - events can get through instead of bouncing the display
back to on state or get ignored when they should not be.

If the transition is being made from states where ui is expected
to be active, do not delay the display state transition due to
ongoing brightness fade.
Do not filter out activity when display is in dimmed state

Although it is normally not possible to have lock screen visible in
dimmed state, it could occur due to some timing glitch or explicit
request over D-Bus. And then user activity is ignored due to tklock
state, display does not bounce back to on state and after the event
eater gets disabled the user interface is active until blanking timer
turns the display off.

If display is in dimmed state, never ignore inactivity.

Also rearrange code to make it easier to see when inactivity timers,
activity callbacks and status change signals get handled.