Headless: Switch to supplied user or previous user after boot.

This adds a system API to specify which user should be switched to
after boot on headless, for use by Automotive devices (for example).

On headless, after boot, the foreground user will be the specified user,
if there is one, or the previous foreground user otherwise.

Bug: 242195409
Test: atest UserManagerServiceTest
Change-Id: I16971511a1303ca718051917c6618655ef0c87b1