New hidden DPM methods: getLogoutUserId() and clearLogoutUser().

Also fixed how the logout user was cleared after a successful switch.

They will be used by CarSystemUi to end a session.

Test: Manual verification with CtsVerifier and TestDpc on automotive

Bug: 205185521
Bug: 204483021

Change-Id: Ieda0a22625ef705fcd705aedfc2dbf2cf570bf81