Consolidate UserManagerImpl::XxxUserLoggedIn methods.

By inlining the code, now UserManagerImpl::UserLoggedIn has three
major blocks;
1) handling multi-user sign in
2) ensure User instance for the login.
3) actual login procedure for the primary user.

This is preparation to move the login flow code to SessionManager,
and also to share the code with fake classes.

BUG=278643115
TEST=Tryjob

Change-Id: I41d53b5de7468a74f792a00621e73b51a35338e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6155998
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1403577}