Move AppStandbyController calls out of UsageStats lock

Move reportEvent, flushToDisk, and dumping out of the UsageStats lock.

Fixes: 158361735
Test: atest com.android.server.usage.AppStandbyControllerTests
Test: atest android.app.usage.UsageStatsTest
Test: manual ("adb shell dumpsys usagestats", with multiple users)
Change-Id: I9f5c2adcb3ef71e82b969b35e7129e715fbfa00d