Write tokenized user interaction extras to usage stats files

Adds support in IntervalStats obfuscation for writing arbitrary user
interaction extras. The bundle keys are tokenized, and then the bundle
is flattened to string and stored in the proto.

Bug: 440630000
Test: manual, checking widget event extras after a reboot
Test: FrameworksServicesTests:IntervalStatsTest
Flag: android.appwidget.flags.engagement_metrics
Change-Id: Id853bd5a7812fdefaa7e16dec8d2493edfd577cb