Don't change bucketing reason if app is already in better bucket.

Bug: 233256741
Test: atest tests/tests/app.usage/src/android/app/usage/cts/UsageStatsTest.java
Test: atest ./services/tests/servicestests/src/com/android/server/usage/AppStandbyControllerTests.java
Test: atest ./services/tests/servicestests/src/com/android/server/usage/AppIdleHistoryTests.java

Change-Id: I9850f160498d5e8a4d0dfa45307fcde9851635f8