Add UnlockThroughputRecorder

This CL adds UnlockThroughputRecorder and moves the unlock throughput
metrics part from LoginUnlockThroughputRecorder to the new class.

Bug: b:328339021
Change-Id: Ieb84ea6663120d80eea1fd3be927ea3b0bd2f45c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5584216


Commit-Queue: Jun Ishiguro <junis@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1311038}