Add Task Root package info to UsageEvent

Bug: 113094946
Test: manual (use "adb shell dumpsys usagestats apptimelimit" to verify
apps at the root of tasks are considered active)
Test: atest cts/tests/tests/app.usage/src/android/app/usage/cts/UsageStatsTest.java#testTaskRootEventField
Test: atest cts/tests/tests/app.usage/src/android/app/usage/cts/UsageStatsTest.java#testUsageSourceAttribution
Test: atest UsageStatsDatabaseTest

Change-Id: I40f86743d33c13892de0e59ae02c9ebddb606ee7