And inform listeners when the bucket changes, not just when
going in and out of RARE bucket.
Avoid redundant callbacks when informing listeners.
Bug: 63527785
Test: runtest -x
frameworks/base/services/tests/servicestests/src/com/android/server/usage/AppStandbyControllerTests.java
Change-Id: Icd98d59f597147fbf8ea4bf44edf4b3b3d5c8e14