Address uses of USER_SYSTEM in AppWidgetService.

AppwidgetServiceImpl has some code that uses USER_SYSTEM as a proxy
for the main user of a device. In HSUM mode this breaks.

Change the backup and restore user to use getMainUser instead.

Bug: 378507338
Test: AppWidgetServiceImplTest
Flag: EXEMPT bug fix
Change-Id: I7b0f79c90af73aecc19ab07a689bd551b822a42b