Implemented INITIAL_USER_INFO.

This property is called by Android when it starts, and it's expecting a
property change indicating what the initial user should be.

During normal circumstances, the emulator will reply right away, passing a
response if InitialUserInfoResponseAction::DEFAULT (so Android could use its
own logic to decide which user to boot).

But during development / testing, the behavior can be changed using lshal dump,
so the following scenarios can be tested:
- property timeout
- HAL response using a different request id
- user switch
- user creation

Bug: 146207078
Test: manual tests with lshal and a modified CarServiceHelper
Test: atest android.hardware.automotive.vehicle@2.0-manager-unit-tests \
      android.hardware.automotive.vehicle@2.0-default-impl-unit-tests

Change-Id: Ia5be62c8b19a168c0c6da5307169fc14bf3069c9