Make UserLifecycleTests more robust to previous fails

If a UserLifecycleTests test failed in a bad way, there could
potentially be users that it created lingering on the device,
which can cause future UserLifecycleTests to fail too. To prevent
this, we make all created test users with the name
UserLifecycleTests_test_user
and we remove all users with this name at the beginning of each test.

Test: atest UserLifecycleTests (including with previous failed users)
Bug: 159650471
Change-Id: I8a652c85fe1d73b2f785c7d16ebe8bfd39ea4710