Refactor observer call in finalizeRestore

There are helper methods for calling the restore observer, but there was
still one place that didn't use it.

Bug: 291249666
Test: atest services/tests/mockingservicestests/src/com/android/server/backup/restore/
Test: manually checked logs when doing a restore

Change-Id: I1f8628c339830a4e9b55243a3fefe4f04ce7bd3e