Supply extra brightness parameters to RenderEngine

A future CL will update the HLG->SDR tonemapping algorithm to consider
current display brightness, as recommended by BT2100.

In preparation for this:
* Fix an issue where maxLuminance was using the current display
brightness if supplied from DisplayManager instead of the max luminance
* Add currentDisplayBrightnessNits to the RenderEngine interface to
support the current brightness
* Plumb current display brightness all the way to libtonemap, where
nothing uses it (yet)

Bug: 206035964
Test: libcompositionengine_test

Change-Id: I3e9f0fdb23fbb08c50e4733e5a16bcd20948d750