After deleting guest user, switch back to previous instead of user0.

In Setting, after deleting or exiting the guest user, or failure to
reset the guest user involves switching back to user0, which is not
possible with HSUM (headless system user mode). With this CL we're
now switching back to previous foreground user instead of system user.

Bug: 283106632
Test: manual
Change-Id: Id60063cca75d9fded7dfe0b7d7a4b871d67c4b0c