Fix NPE during migration.

User restrictions are null for PO parent admin, so we need to ensure
they are initialized before adding.

Bug: 147884539
Test: atest com.android.server.devicepolicy.DevicePolicyManagerServiceMigrationTest#testCompMigrationAffiliated
Change-Id: I5024ac585e30081f944717cfe8d4b920b644213e
3 files changed