Listen to changes in unused threshold

CTS tests were failing because some classes were using a cached value
for the unused threshold, causing apps to be used when we wanted them to
be unused under test. We register a callback for when the property
changes to update the value to fix this.

Bug: 188092696
Test: atest CtsOsTestCases
Change-Id: I7dea406738d42f286436a31640a4bd626ad7d369
1 file changed