Return correct splines from ddc calls

Methods in the displaydeviceconfig will now return the appropriate
splines based on whether the extended low lux range curves are
available. If the even dimmer setting is turned off, the minimum
brightness will still be capped at the nits level it would be
without this feature.

Bug: 179428400
Test: dumpsys display
Test: atest DisplayServiceTests
Change-Id: I2dc0b2b855bb1f590d19b9e89d29d6c703c1a676