ash: Move VpnService's SetParameters out of crosapi

To facilitate this without major changes, make the crosapi
implementation provide a GetActiveConfigurationObjectPath method and
call that directly (not via mojo). This will eventually be deleted
together with the rest of the crosapi implementation.

Bug: b:365741912, b:365902693
Change-Id: I4c48d3bb2faac5788e05a0284d8cd5bc1668e1c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7005514
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1527322}