Provide transition duration for ui side dimming

The enabler for ui side dimming was already implemented earlier, but
turns out it does provide enough information and had some glitches.

In addition to fade-to-black -level the signal contains also the
transition duration that ui side should use for the level change.
This allows using mce brightness fading timing configuration also
for the transitions happening at lipstick side.

To avoid unwanted brightness jumps during display power up, the ui
side fading level is retained over display off periods. Whether it
is still needed after display power up is evaluated when als data
becomes available.

Fading by 70% made the display too dark - adjusted to 50%.

[mce] Provide transition duration for ui side dimming. Contributes to JB#18412
Merge pull request #220 from spiiroin/jb18412_ui_side_dimming

Provide transition duration for ui side dimming