Offload Luma Sampling IPC to background thread

It's observed that, in the field, SystemUI is ANRing while trying to
register the luma sampling events.

We can offload those to our background executor to keep this from
happening.

Bug: 193922859
Test: manual
Change-Id: Ic5e6c5efa33a166f2e3d81eb12756f606365ae9a