Unify MultiCaptureNotification and MultiCaptureLoginNotification

This CL implements the feedback provided on a previous CL [1] and
unifies the two multi capture notification classes, i.e.
MultiCaptureNotification and MultiCaptureLoginNotification.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/4400990/comment/14f0d264_707b9782/

Change-Id: I6d2d70ccb83f1c0ab99f66d23b0ee8fe2e205e4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4637268
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Simon Hangl <simonha@google.com>
Cr-Commit-Position: refs/heads/main@{#1164055}