histogram: refactor histogram thread

refactor the histogram collecting thread from being an event
loop based on the control node to being a bottom-half worker
for events sourced from the master node.

cherry pick from hardware/qcom/sm8150/display

Bug: 143513619
Test: ./color_sampling_test
Test: libgui_test  --gtest_filter=DisplayedContentSamplingTest*
Test: dumpsys SurfaceFlinger

Change-Id: Ic1366ccafab0491d4238d162c40c81d8b65dd275
3 files changed