Arc Nearby: Send files through Intents using filesystem: URLs

Arc Nearby currently passes files as file: URLs through Intents, to
allow sharing files that are stored outside of folders known to File
Manager.

However, as of crrev.com/c/3337403, Arc Nearby shared files are now
stored in a directory managed by File Manager. This allows us to instead
use filesystem: URLs, which is more consistent with the rest of the
Intent system, and simplifies the Intent encoding and decoding in Nearby.

Bug: 1253219, b/201588599
Change-Id: I0438e8b4a2a749f5bfdf48d0f84dc965e5e321fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3362002
Reviewed-by: Josh Nohle <nohle@chromium.org>
Reviewed-by: Alan Ding <alanding@chromium.org>
Commit-Queue: Tim Sergeant <tsergeant@chromium.org>
Cr-Commit-Position: refs/heads/main@{#955557}