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 an restore user to use getMainUser instead.
Document that the other two uses explicitly refer to the system user.

Bug: 378507338
Test: AppWidgetServiceImplTest
Flag: EXEMPT bug fix
Change-Id: I1bac66711a37f604bc46656e2382faa39eaa1abd