SF: rate-limit luma sampling

Instead of sampling the luma regions every frame, introduce a rate
limiting system to reduce load. Introduces a few tunables to control
the rate, which is defaulted to 10Hz, when there is content being
watched for luma.

Test: manual systrace inspection, using SamplingDemo
Test: libgui_test --gtest_filter="RegionSampling*"
Test: atest CompositionSamplingListenerTest
Fixes: 126747045
Change-Id: I7cae3e90fb405ba72dc2f276a88be48f1533a219