Populate null UsageStats event task root fields with known data

It is possible for an activity's task root to be reported as null to
UsageStats during a destry. Use past known data to populate the fields.

Change-Id: Id20c809580e854ca9075444957bd10a338087e3d
Fixes: 123404490
Test: atest cts/tests/tests/app.usage/src/android/app/usage/cts/UsageStatsTest.java#testSuddenDestroy