[FilesTiering] Pin items in batches after space calculation

Once the space has been calculated, bulk pinning can be commenced. This
simply pins items with no additional space verification nor does it
verify the item was actually pinned (setting the flag to pinned vs.
actually getting pinned are 2 different outcomes).

In an upcoming CL the above problems will be solved.

Bug: b:259454320
Test: chromeos_unittests --gtest_filter=*DriveFsPinManager*
Change-Id: I7f2e2daae5605200f4733e5f2693091ba4669fb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4060674
Commit-Queue: Ben Reich <benreich@chromium.org>
Reviewed-by: Bo Majewski <majewski@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076666}