Changed DevicePolicyUsersPreparer to use SwitchUserTargetPreparer.

The DevicePolicyUsersPreparer depends on the initial current user
being a full user on HSUM devices, so it should not rely on clients
to set that user.

Test: atest CtsDevicePolicyManagerTestCases:DeviceOwnerTest#testProxyPacProxyTes
Bug: 420640007
Bug: 435528858
Flag: TEST_ONLY
Change-Id: I72e23911f62291a9a72f45f729ff1873493a9416