Fix bad test for testGetUserForAncestralSerialNumber which was

previously marked as @Ignore.

The test was bad because it set 11 as the ancestral serial number for
the system user UserBackupManagerService but asserted that 11 corresponds
to user 1 - which is not the system user. This CL fixes the assert.

Also added corresponding test for non system user.

Fixes: 147012496

Test: atest com.android.server.backup.BackupManagerServiceTest
Change-Id: Iab736885264aa4befc644678e5fe66d602ed00e3