Fix NPE in AppWidgetServiceImpl#maskWidgetsViewsLocked

This CL added following changes:
1. Included fallback logic in maskWidgetViewLocked in case
   AppWidgetProviderInfo#providerInfo is null.
2. Explicitly asserts AppWidgetProviderInfo#providerInfo is non null
   upon assignment in debug builds to reveal the root cause why it was
   null in the first place.

Bug: 339669351
Flag: NONE
Test: manual
Change-Id: I2676a62b1a6bc6c24df6900bde09711003200df5