[Launch Lacros at login screen] Readahead Lacros files

This commit is part of a series, meant to provide the capability
of pre-launching Lacros at login screen for performance reasons.
Design doc: http://go/lacros-loginscreen-design

Previous relevant CLs:
- http://crrev/c/3793529 (Pre-launch Lacros (under flag))

This change adds an additional optimization to Lacros's pre-launching.
Given that Lacros blocks relatively early at login screen, it doesn't
read all the files that will be needed later in its lifetime before
it hybernates.

Similarly to what happens to Ash Chrome files at boot with ureadahead,
this commit preload the corresponding Lacros files (only when the
preloading flag is on). From preliminary tests on Careena DUTs, this
change can shave up to additional ~0.7s of startup time (as measured
to the moment when the Lacros window is fully rendered).

Bug: 1363933
Change-Id: I19dc56d9656783050f0e1773c00805d2079fdd05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4042122


Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Andrea Orru <andreaorru@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076563}