Remove any existing users in test setup

Remove any existing users in before running the tests to
avoid failure when trying to create a new profile if one
already exists. This logic is mainly copied from
BaseDevicePolicyTest#setup

Fixes: 155355517
Test: atest ContextCrossProfileHostTest with and without an existing
profile.

Change-Id: I38187be64b26cdd48a1e5dd14902964bf2364352