End-to-end plumbing for dimming SDR layers

Model here is:
* HDR luminance is set to the current display brightness
* SDR luminance is set to the current SDR white point reported by
DisplayManager

Ideally we use scene-referred white points instead, so:
* PQ is always 10k nits
* HLG is always 1k nits
* Everything else is 150-200 nits

So relative dimming thresholds are fixed. But right now this is visually
less jarring (otherwise youtube UI will suddenly dim when autoplaying
HDR video).

Bug: 200310158
Test: Verified that plumbing sdr white point is sent to renderengine
Test: librenderengine_test
Test: libcompositionengine_test
Test: DataspaceUtils_test

Change-Id: I5bcea7941935c43e57cd5434e1ec69b41d31f2b4