Fixed DPM.logoutUser() for headless system user mode.

On "traditional" devices, this method switches back to the "primary"
user, which is the system user. But on devices running with headless
system user mode, it should switch to the previous user instead.

Test: manual verification (TestDpc/CtsVerifier) on car and phone
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageUser_LogoutUser
Bug: 204483021

Change-Id: I3ccbc1f31f8bcc4dae8fcb71cf5de7a7ad8882fe