Update the timing to unblock post-login tasks.

Currently, post-login tasks are unblocked when all tasks considered
critical to login are completed, including animations.
However, due to race condition, the event may not reliably emitted.

This CL moves the timing a bit earlier, so while there're some
overlap of the timing to run the post-login tasks, and the animations,
it should run.

We also take the new metrics BootTime.Login4 for this purpose to see
the impact of animation waiting compared to BootTime.Login3.
Once we confirm that has negligible impact, we can simplify it
by replacing it to BootTime.Login4.

BUG=b/326065955, b/323098858
TEST=Run locally.

Change-Id: I6cfa234116d5b7ed9e3d71f3451ec7417f26a762
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5348944


Reviewed-by: Jun Ishiguro <junis@google.com>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270646}