Use taskRootPackageName instead of packageName to load uid when usage

resource is USAGE_SOURCE_TASK_ROOT_ACTIVITY and taskRootPackageName is
not empty.

This logic is consistent with digital wellbeing: assign the screen-on
time onto task root activity when usage resource is
USAGE_SOURCE_TASK_ROOT_ACTIVITY.

Bug: 260964679
Test: make RunSettingsRoboTests + manual
Change-Id: I4c7ed342d8c00951879f5826bf79575f330ce86e