Add system API to get the previous user.

This adds a system API to retrieve the user that was last in the
foreground, apart from the current user.

This may be used for example when the current user is removed,
to switch to the previous current user.

Bug: 259373662
Test: atest UserManagerHostTest
Change-Id: Ie21fe2f47a90633d2e2a9cb36f03d7d1bca1f9d5