Customize user switch dialog

- Introduced DevicePolicyManager to set messaging for switching out of and into user 0
- Introduced coressponeding API to get the message

Bug: 71787514
Test: Set start and end session message with TestDPC, change is reflected
Test: Restart the device, switch message is still enforced
Test: Clear the message with TestDPC, reverted back to default
Change-Id: I527eca6f151cee35459abad7ae9dcdeef486148f