Cleanup to composer apis for HDR:

1. Remove the setDisplayBrightness method, as it's been moved to a
display command
2. Rename the WhitePointNits stable stuct to Luminance, since it's a
more general descriptor.

Bug: 210151839
Test: builds
Change-Id: Ic948dce737ce718772100b0e976ec93069d5e8e1