Handle STOPPED to PAUSED Usage events

UsageStatsService wrongly assumed a ACTIVITY_PAUSED event must be
preceded by a ACTIVITY_RESUMED event, but there are some scenarios where
it is possibly to go directly from STOPPED to PAUSED.

Fixes: 182007710
Test: manual (start a pip activity, turn off the screen, and turn it
back on. UsageStats logs should not report unexpected events.)
Test: atest android.app.usage.cts.UsageStatsTest

Change-Id: I609ecb574d8401ab597aa61ba7c3b9d1f4e66827