Fix screen time not updated issues.

When slotUsageData is null, the UI should update screen time to 0
instead of skipping update.

Bug: 304432148
Bug: 299801782
Bug: 301943900
Fix: 304432148
Fix: 299801782
Fix: 301943900
Test: manual
Change-Id: I25da1abf8ea79305ebf5468977b2a57ce917609f