Resolve Test flakes

-> Synchronize calls in LoaderTask.java
issue 156041043

-> Remove non system users on setup
If a work test crashes before getting to run its teardown - we might end up with a user profile that could throw off subsequent tests.
issue 156022161

Test: Manual
Change-Id: Ife708a3de01572f7cb2187078d592d8d570dd951
2 files changed