Consolidate launch time change notifications.

Track pending estimated launch time changes in a single Set so that we
avoid sending repeat notifications for a single package.

Bug: 227796747
Test: atest CtsUsageStatsTestCases:UsageStatsTest
Test: atest FrameworksCoreTests:android.util.SparseSetArrayTest
Test: atest FrameworksMockingServicesTests:PrefetchControllerTest
Test: atest FrameworksMockingServicesTests:UserUsageStatsServiceTest
Change-Id: I671c97e4e4110d0a1c76a8f6400dfe8c1e5055dc