Listen DeviceConfig changes

DeviceConfig reads are not free, and AttentionDetector code sits on a
critical system path. Avoid reading DeviceConfig on each invocation,
add a onChangeListener instead.

Bug: 169427927
Test: manually and atest AttentionDetectorTest
Change-Id: Icb89f78f07207ee8e80a11f16b0f03ab578f4ea6