Fix the unit test UsageStatsTest.

Entry is removed from mActivities after event ACTIVITY_STOPPED, unit
test needs to be fixed.

Change-Id: I8ecf2eb75c694478e940fa82a34c6e6948847d96
Fix: 141953236
Test: atest  frameworks/base/core/tests/coretests/src/android/app/usage/UsageStatsTest.java