Refactor DPMS tests to be able to test more different cases.

- DpmMockContext.addUser() allows to create more users.  This will set up
UserManager properly and create a data directory.

- Updated DevicePolicyManagerTest so it can now easily emulate calls from
different secondary users.

Change-Id: Ic9d3e8d0d1e7f41e184c82e6dc323c244535b8d8