multipaste: Return unserialized clipboard history items from controller

This CL changes the clipboard history controller's GetHistoryValues()
API to return strongly-typed clipboard history items rather than Values.
Doing so eliminates the possibility of future key mismatches (as was the
case in b/264696776) and makes it easier to integrate with more surfaces
outside of ash, such as the browser.

Fixed: b/266947643, b/252366283
Change-Id: If80aad1265ec3e4e47166c94d218d83c739cfe4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4307338
Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: Colin Kincaid <ckincaid@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1115182}