Remove concept of nits from the client target properties

An earlier patch replaced white point nits with a per-layer brightness.
This patch does the same for providing the brightness space of the
client target relative to the display brightness.

Bug: 217961164
Test: builds, boots
Change-Id: I1be65f7c511fefa239305e0735637126a1cd6622
Merged-In: I1be65f7c511fefa239305e0735637126a1cd6622