Allow standby timeouts to occur after usage

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