Stabilize libgui RegionSamplingTests

libgui RegionSamplingTests fail on physical devices due to
incorrect sampled luma values. The incorrect region was
being sampled due to other active listeners that were not
cleared. Modify test API to get all existing active listeners,
cache them and clear the list to create a blank test slate, and
re-add the cached listeners to restore state after a test is done.

Bug: b/365708032
Test: atest RegionSamplingTest in libgui_test, libgui_test
Flag: EXEMPT, test fix
Change-Id: I70eb3bae070174b7db87ee16d8343bbc1e81bad0