[Nearby] Update sharing::mojom to use absolute namespace

This change explicitly states the absolute namespace for ::sharing, as
there were conflicts with using that namespace and ::nearby::sharing.

Test: Manually verified Quick Share and Phone Hub still WAI on DUT.
Fixed: b/336347986
Change-Id: I2071c4a14104a4383e368bc1fb68ef0d3068e88d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5473642
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Crisrael Lucero <crisrael@google.com>
Cr-Commit-Position: refs/heads/main@{#1290862}