Don't try to create user for task snapshot tests

Seems user creation can fail in test harness some times.
Also, I am not sure there is a need to create a user in this
case when we can just use the user id of the current process.

Also removed SettingBackupTest from @Presubmit since it is still
failing.

Bug: 37684646
Bug: 37682538
Test: SettingsBackupTest
Test: TaskSnapshotPersisterLoaderTest
Change-Id: I523bb576217f7782027741916f9ea14fc6d7fb50