dlp: Request file access when pasting files

Currently we are not properly requesting file access when pasting
files. As a result, lower level checks are performed per file and
multiple notifications asking whether the action should be proceeded
are shown to the user in warning mode. Block notifications are instead not shown at all.

With this CL we request file access before the lower level checks are
performed, so that the proper UI can be shown to the user.

Bug: b/307223052
Change-Id: Id415b272f0ae1997a1d00815adb12e46314538d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5054080
Reviewed-by: Aya Elsayed <ayaelattar@chromium.org>
Reviewed-by: Daniel Brinkers <brinky@google.com>
Commit-Queue: Luca Accorsi <accorsi@google.com>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Austin Sullivan <asully@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1232459}