Wait user switch complete at the end of startActivityValidUser

switchUser is not a synchronized operation. That may cause to
remove user in tear down before the switch to original user
is completed, and system may meet unexpected state by user
not found.

Bug: 165771802
Test: StartActivityAsUserTests#startActivityValidUser
Change-Id: I0d03b691ba32e16e3513dce991ce26ced94f2514
1 file changed