Put the current user to mUsers in UserManagerServiceTest.

Because of the way we construct UMS from UserManagerServiceTest, it
can not populate mUsers with existing users. Because it can not find
userlist.xml and fallback to single user.

And the test methods which try to set a restriction for the current
user fail because UMS.setUserRestriction fails with "Cannot set user
restriction no_user_switch. User with id 10 does not exist." error.

We're fixing this issue by manually adding the current user to
UMS.mUsers via mUserManagerService.putUserInfo() method.

Bug: 289517147
Test: atest UserManagerServiceTest
Change-Id: I6d17fe1fc34408acde8f6b7233a7cf96a5dbfa45