Add multi-user test for the general non-staged case

This test makes assertions about the case where there are two
users running a test application. It verifies that each user can
have its own unique userdata for the same app, and that upgrades
and downgrades are applicable for all users on the device.

Test: atest MultiUserRollbackTest
Test: atest RollbackTest
Bug: 140401995
Change-Id: I5e5c154758b3f0e4731696c813519ac798fbbae9