No need to switch users before running tests (2/n)

My local test shows that switch-user doesn't work on my physical device.
In fact there are APIs to run tests against a particular user without
switching. It also speeds up tests without switching users from time to
time.

Bug: 149876119
Test: atest MultiUserRollbackTest
Change-Id: I34d26ddcb6a6e9cdc39228310830a3cd83212e4a