[lensoverlay] Change UKM metrics to use enums.

This changes all the UKM metrics to only be recorded once at the end
of the session. It also changes the metrics to use an enum to split
by page content type instead of listing every page content type in the
metric definition.

Bug: 382080022
Change-Id: Ib6f89547b1b81cf28d24801c3a551c21ab501d81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6065413
Reviewed-by: Sun Yueru <yrsun@chromium.org>
Reviewed-by: Ali Stanfield <stanfield@google.com>
Commit-Queue: Nihar Majmudar <niharm@google.com>
Cr-Commit-Position: refs/heads/main@{#1392928}