Persists restrictions in XML before marking user creation completed

To prevent user restrictions from being lost in the event of a system reboot immediately following user creation,
save user restrictions into it's XML file before setting the userinfo.partial flag to false.

Bug: 263308383
Test: manually build and tested the user creation flow
Test: atest UserManagerServiceTest#testDefaultRestrictionsArePersistedAfterCreateUser -c
Test: P66097023
Change-Id: Ied90ca38780a472eef0f66b8f53089a2a3202e2b