Move isUserSwitcherEnabled tests to mocked UserManagerServiceTest.

In servicestests.UserManagerServiceTest, isUserSwitcherEnabled tests
are trying to set some system properties, but it is failing with
"SELinux permission check failed" error. This CL moves these test
methods to mockingservicestests.UserManagerServiceTest and mock some
of those dependencies, and properly test UMS.isUserSwitcherEnabled
method without failures.

Bug: 264667464
Bug: 264667776
Bug: 264666926
Test: atest UserManagerServiceTest
Change-Id: I7a5c2becfbab023bdbb2d5f5b3a1415a28225be2